Hello there,
after having played the game for about 40h I want to share some ideas that I think would make Infraspace a better game 
I mostly play on what I consider the hardest difficulty: hard demands, hard transport efficiency and round trips. This is important for some observations. The following may contain some spoilers, read at your own discretion if you’re new to the game.
Habitats
Make habitat upgrades cost building resources.
The way habitats work is very similar to Anno (at least 1602). There upgrading your settlers houses costs building resources and this would also fit with Infraspace. Turning your meager space huts into tall skyscrapers shouldn’t be free and the cost should raise sharply with each level. This would also tax the construction material industry more in later stages of the game.
Make inhabitants produce waste.
For each x-th unit of food (and water?) consumed, habitats should could create bio waste to be turned into methane later. Also consumer goods should turn into some form of scrap which can be brought to a building (I’d call it Recycling Center if that wasn’t taken already 
) and then maybe converted into a random unit of metal. Depending on the ratios, this could make a considerable amount of extra resources available for large settlements and make citizens feel less of a burden in the hands of a capable player. Also I think the theme of circular economy and recycling makes absolute sense in this game. No civilization will master interstellar travel without being able to achieve recycling rates on board their ships of well above 99%. Also it would create more circles in the production graph of the game (like with industrial robots and high tech tools), which makes production planning a lot more interesting, especially with round trips enabled.
Habitats show their power consumption on inspection.
When I click on a habitat I want to see how much power it currently consumes. Because it does need power but it doesn’t say anywhere how much. Would be nice if you could just tell me 
Trains
Make rail tiers more visually distinct.
Sorry, but the tiny arrows don’t really cut it for me 
I would like to see some greater visual differentiation of the rails, comparable to roads. For example the first tier looks very simple, almost like ordinary rails. The second tier looks like the current tracks and the lightning rails have some fancy glow to them or trains leave trails of sparks or something that justifies the (awesome) name.
Provide an upgrade for train acceleration.
From what I can tell the three tiers are only different in max speed but not in acceleration. So in my design I have main loops of tracks and branch off for stations. Now after each station I need a ludicrously long track for trains to let them accelerate before I let them into my main course where I only want vmax trains. If there was an upgrade for acceleration I’d totally buy it! In general I would love to see more individually upgradable things in the research trees, like wind turbines and solar panels already have.
Allow sorting of train routes and train stops within routes.
Some drag and drop mechanism to rearrange the order in the lists of the train menu would be really convenient. If I want to insert a stop into a list I basically have to redo it from scratch.
Trains show which route they belong to.
So far I can only assign train routes colors but those aren’t always easily distinguishable plus I can’t remember my color scheme very well anyway. I think there is enough space on the small info box of a train when I click on it to also tell me which route it belongs to.
Trains and train stations show the amount of return cargo.
With the return trip option enabled, the game is so kind to show me in stations and trains that such return cargo is being transported, but not the amount. It would be helpful for optimizing my infrastructure to know how many are waiting in the stations and making up train cargo when I inspect them. Maybe put the number in brackets to indicate that it’s not the actual freight. Or something like that.
More detailed train route settings.
Trains are one of the most satisfying concepts in the game once you get them working for you rather than against you. And then you crave for more. I would like additionally settings for my train routes, for example limit what can be loaded and unloaded at certain stations (districts don’t help with stations) and how long trains are for starters. Also an option similar to traffic lights would be useful: “don’t stop at station if nothing to do”, because station empty or train full or whatever.
Allow change direction of rails inside train stations.
I usually don’t need opposing rails in my stations, they tend to all go in the same direction. Maybe the problem is with me here, but I think let the players have as much freedom with their designs as possible.
Transportation
Enforce return trips.
This is the most important one to me. As the tooltip explains, enabling return trips of each cargo greatly increases the difficulty. I was pleased when I learned the hard way that this is no idle threat. But I was disappointed when I found out that return trips are optional: If there is no return route then they just don’t happen. Now it’s easy to design most of your infrastructure in such a way that no return route exists and the challenge completely goes away. In the future I will simply force myself to make sure all returns take place, but it would be much better if the game forced me to. For example, make it so that a good delivery is only started, if a return trip is currently possible.
Selected cars (and trains) also show the route from their origin to the current position.
The trail cars plan to follow from their current position to their destination is really useful for debugging roads. It would also be useful to see where they were actually coming from and how they got to where they are right now. I suggest to show in a different color (e.g. blue) the path the car has traced so far and keep the orange for the future path.
Allow customizing the default intersection settings.
This is not for casuals so I understand if you don’t even want to consider this. I often find that the default intersection settings (in particular which lane can go where) do not match my typical needs or are outright suboptimal. So I would like to change those defaults, lest I have to manually modify each and every road intersection by hand.
Production
Calculate the demand for industrial robots and high tech tools.
In the production window I would like to know how many of the building upgrades I need to produce. You know which building benefits from industrial robots and the cute purple calculators (high tech tools). You know how long they last. With this information it should be possible to calculate the demand and display it, at least once the tech is available.
Take recycling centers into account for demand calculations.
Whenever a building is under the influence of a recycling center, it’s demand is cut by 1/6. This 16.7% reduction in demand should be reflected in the production window when summing up all consumers’ total demand.
Buildings with a steady output should not get an efficiency bonus.
There are two types of buildings this pertains to: fuel consuming energy buildings and the terraforming buildings. They consume resources but produce a steady output, not a good. If they are built next to input producers they gain an efficiency bonus, but this only makes their resource consumption faster without producing more output. Either scratch the efficiency bonus or actually give me something in return when they run at more than 100%.
Construction material only gets sent to the storage if no other consumers can be reached.
With the way goods are delivered in the current system, it can be difficult to get a proper distribution of goods. By slightly outproducing your consumers this can be alleviated after some time because the preferred consumers (i.e. those closest to the producers) will have 40 units in stock and thus the next closest consumer will start to receive more goods. The problem is that the construction material storage has an infinite storage, so if you put say a concrete consumer further away, then it will never get any concrete. Got me pretty baffled in my first game. So I suggest to give the construction material storage a special lower priority than all other consumers of construction material. Or make the distribution of goods smarter or more fair in general (can’t say I would really like that though).
Increase the usefulness of large mines.
Well, I really like large mines and they are useful when you can research them. But with the advent of industrial robots they are not all that special anymore except for using less footprint.
Increase the usefulness of pipes (and tanks).
In general pipes are awesome. They make your life so much easier. Until you learn that they cut your buildings’ efficiency by a whopping third. Since I consider my labor really precious and by using pipes I need more labor, I always opt not to use them. Trucks are driven by robots or remote control or something, so let them haul the stuff on the roads without incurring additional labor requirements. If the drop in efficiency was only 10-20% maybe it would be at least worth considering to lay down the pipe networks.
Miscellaneous
Solar panels are overpowered.
In general the balancing of the game doesn’t seem very important to me, but solar panels are definitely an exception. Once I got those I won’t build other power plants, in particular not those that consume resources. Because those need labor to run, and more labor to run the mining. With labor being the most precious resource in my opinion (more so with “hard” difficulty for citizen demand) solar panels are clearly the only option. Besides being decentralized. There are different ways to tackle this. The easiest but probably not best way would be to make solar panels also use labor. For the methane power plant I have a better suggestion to make it competitive: Have it produce carbon as a by-product. I’m pretty sure that the atmospheric processor and the carbon processor are meant to get carbon from CO2 in the air. Well, if you’re burning CH4 then why not capture the emitted CO2 to run it through the same process, which should be much more efficient with this setup due to higher concentration of CO2, to get the Carbon out. Could potentially be a research-able upgrade. Carbon is a very useful good in this game after all. For the nuclear power plants I don’t really know.
Make more use of the space view.
I think there is room for a lot more sub menus when you are in the space view. Next to the ship I think it would be really cool to also have other options for building things in space, e.g. launching satellites, asteroid mining operations and other space facilities. The main focus of the game should still be on the ground, but having a few more things to do up there in orbit would lead to a whole new sub-tree and building category that the player can explore.
Give us copy+paste.
I think this was mentioned in other places before. Copy+paste of buildings or blueprints would be nice.
I probably forgot some points and will post additions if they come to my mind. I hope this post was useful for the further development of the game 