A little bit of context:

I am part of an online community that focuses on the artistic movement known as Speculative Evolution, and as the name suggests, it is a genre of sci-fi that focuses on the hypothetical evolutionary scenarios, including my favorite: xenobiology. For excellent examples of this being done well, Google "Snaiad" or "Sunrise on Ilion". As such, I had an idea for a game in which one is a colonizer on an alien system where one needs to build settlements around the planet in order to ready for the second wave of colonists, which are just ordinary citizens. 

Game:

You are a the head colonist in the first wave of colonization of the Errante System, which has an Earth-like planet that has been targeted for colonization.

One starts on their spaceship, which is equipped with enough resources to last several in-game weeks. The player has a single landing pod to use to drop off supplies and/or other colonizers to help with colonization and start with a single randomly-chosen location to start colonizing. As the player progresses through the game they earn more complex materials to which build their settlements with, more advanced tools for mining, access to more advanced life-support systems, more locations to start colonizing, upgrades to the mother ship, etc. The end goal of the game is to survive until the second wave of colonists arrive, ending the first part of the game and launching the player into free-play mode, where they have the ability to travel to every location on the planet they have yet to unlock and gain a massive amount of funds by the government of Earth to continue the colony world. This is the very basic idea. Eventually, I'd love t have three different worlds being able to be played, each with its own unique biota and challenges (easier planets are Earth-like while more challenging ones are desert planets), and a story that connects all three worlds with the colonist being played by the user having a kind of designated role like a scientist, a military leader, etc.

Now, my question here is what kind of mechanics should I add besides the ones mentioned, or which ones could use more expansion on? Thanks in advance for any responses.

Hello!

I like the idea very much! One way I can image this is like some sort of sci-fi version of Kingdom.

One idea which just came to my mind: How about spherical (or rather, circular) planets? Like the surface of the planet is the edge of a circle, and while the player moves around the planet, the camera is rotated so that the bottom edge of the viewport always points to the center of the planet.

One problem is, on large worlds, the visible effect would be small or even unnoticeable (flat Earth conspiracy anyone? ?). But maybe you could add sequences where the camera zooms out so far that the curvature (or maybe the entire round planet) becomes clearly visible.

Or, if not, you could just make the world indeed flat, but in a way that going right all the time means arriving at the very left again at some point, and vice versa. However, a truly circular world would be cooler imho.

I had thought of doing this in that matter but eventually, I decided against it because of one main thing, which is that it limits the scope of the game to a single longitude or latitude. Say we kept the user at the equator. Then hot biomes would all that would be exposed to the user, no taiga no plains, etc. If we kept them at the Prime Meridian then they would miss a ton of continents, or I'd have to modify the maps of each planet to fit so that the user can effectively go through the planet and experience as many biomes as they can. Instead, I have them unlock areas of the planet from their space ship, like, a section of the Great Iredian Desert or the Serdoni Rainforest, as examples.

I don't see a problem. If you make the surface a great circle through the poles, it will go through all latitudes, and even each latuitude four times (north-east, north-west, south-east and south-west). So you could have sea, low land, high land and even one more "bonus" altitude/terrain at each latitude/climate zone. Practically even a lot more, since the limit of four only refers to every *exact* degree of latitude. Just slightly further north/south, you can have any variation you want. 

And of course your one-dimensional game world would only represent a tiny section of the whole planet if you imagine the planet as a 3D sphere. However, first, the same is true for your separated areas, and second, I think it's a wrong way of thinking right from the start. Your game takes place on the 1D surface of a 2D planet. In this world, lands beyond this line are not inaccessible. They simply don't exist.

And with a circle planet, you could add cool things like orbiting satellites, seasons and circumnavigation of the world, all in a consistent, "realistic" way. 

Please make a circle planet!

Assuming the planet isn't ridiculously small (and given the colonization and exploration premise it shouldn't be) latitude, longitude, great circles, curvature of the surface and other such geographical features are irrelevant at the small scale of a "2D side-scroller" featuring astronauts, buildings and small vehicles.
Levels would be simply some path between location A and location B (or the interesting, adventurous portion of such a path); probably a good fit for procedural generation of planetary maps, levels between map landmarks and features inside levels.

The only important feature the setting suggests is interaction with aliens, probably sentient and split between friendly and hostile groups, with advanced interaction possibilities like trading, conversation and fighting as allies. 

Can you explain what sort of 2D side-scrolling action you have in mind? Most game features you describe seem a much better fit for a 2D overhead perspective and (probably) an open-world structure:

• exploration and search (as opposed as reaching the end of a linear path)
• placing and managing an increasing number of buildings
• transporting resources
• handling many colonists (as opposed to a single hero)