I know it's probably very early to think "expansion" but it might be important to implement certain game systems in vanilla so they are available in the future.
Anyway, my idea for an OTC expansion is: Offworld Terraforming Company.
Who doesn't want to see the red Martian landscape transform gradually into lush forests rivers and oceans.
Alright, now that I'm home here is a more fleshed out vision for this expansion idea:
First I think that the abrupt way a game ends is probably not as fun for those who lose. I think an option to use the proceeds (or a prorated fraction of them depending on game balance) from when you get bought out to re-buy back in with a fresh colony would keep you interested in a game you'd all of a sudden lost. Comebacks should be possible even once your bought out! The game would end when someone's net worth reaches a certain level determined by all players at the start of the game. Alternatively if you like this terraform Mars idea, the game could end once a certain level of terraforming is complete and a countdown estimated timer could be shown till the end.
Next, I would like the npc colony to have a larger impact rather than just use up resources to affect the market. I see the main colony as a government organization actively trying to make Mars more like Earth. To that extent it could offer missions to players in game that could be bid upon that offered rewards once completed. Examples could be "strengthen the electromagnetic poles of the planet through core drilling (requires large investment of steel and (new resource) uranium or or increase atmospheric pressure to a certain level (requires H2O, O2 and maybe C02).
With these missions in mind, players could select a focus for their company such as subterranean, atmospheric or biological (or work towards all three!). Subterranean would be able to cause things like earthquakes and volcanoes that could raise and lower then land on the map, expel ash into the atmosphere or damage buildings of opponents.
Atmospheric could cause heavy rains that would create rivers, lakes, and oceans and could even flood an opponent's base of operations lowering production. Additional options include lightning storms or heavy winds that could affect power production.
A biological focus could allow players to create microbs or other fauna to promote O2 or fuel (H2) production or might act like pleasure domes and increase tourism to the planet. Food and water would be a resource that would be in heavy demand for biologically minded players. Subterfuge opened up by this path could include "microbial pandemic" that creates food shortages or "animal stampede" that could selectively hinder opponent's operations. An algae bloom could start to color the map green!
All three of these paths would be opened up depending on building/technology pre-reqs being built. Similar to the hacker array or offworld trade center, or patent lab. The key would be to figure out which areas were underrepresented by other players and which tasks the npc colony was most in need of to terraform Mars into a sister Earth.
I'm sure I and everyone else will have many more ideas once we actually get to start playing OTC on Thursday, so once I get a feel for the base game I'll post more ideas for an expansion's potential direction.