So I've been recently playing another well-known space 4X (with narration voiced by a certain famous Star Trek actor, to not name either). A simple but nice idea that stuck with me was that each race was allowed a special terraformation process, only for their homeworld's planet type, to be unlocked through research. That Terraformation turned the planet into one of comparable features but generally being much more hospitable, hosting more population, allowing more growth, etc...

While simply copying the idea would be unoriginal and, I think, ultimately bland given ES2 couldn't have as much diversity in homeworld types, except for specific species (and that's quite alright), I believe there's potential in terraformation 'beyond' the Tier-0 planets or other types. Such upgrades could help revitalizing garden worlds late-game when their populations has peaked, or maybe this could help create specialized planets other than Gas giants.

For example, Coruscant-like mega-cities spanning entire planets could offer a specialization in trade, automatically hosting a bonus Trade Company and increasing income from all routes passing by it. Ark planets covered in gigantic natural reserves, while not offering much in term of population space, could be engineered to host the galaxy's rarest species or enact immense experiments, boosting science, empire-wide Approval or even ecologist political support.

Overall, I think this would be a great end-game development for peaceful empires. Maybe super-planets could be wonders in regards to a victory condition. This could also expand upon the unique planets of the galaxy, which could give their best bonuses once restored to their original state (for example, restoring Auriga to Ark world status).

Interesting, and terraforming options will be much more diverse already because Amplitude has done such excellent work increasing the number of, and giving awesome names to planet types. This emphasizes the incredible complexity that I loved about the first game.