Mega Structures. Things like Dyson spheres, Shipyards, Production Arrays, Trade hubs (That can allow for cross-region trading with the “regional trading” skill). etc. They can provide insanely high bonus’s for specific things, but alliances are limited in the amount of them they can build (maybe even just 1)
More things with planets, including neat things like terraforming, or just growing population on it. It might be more interesting to push pi in the direction of causing players to have to build cities, which in return employ and produce goods. This will help push the need for more pi interaction, helping make it more of a focus game play concept (more full time). You may even look into actual direct isk generation. Make a colony, and then grow its population for “taxes” as a “governor”. It might be even better if you can some how directly link planets together.
Bonus: Create a graphical animation of small lights to demonstrate shuttles of the population of the planet to represent population / activity on the planet.
Stellaris does this also here is an example (You can see the shuttle lights on the left side/dark side of the planet).