TL;DR
Add some researches with Prometheum science that are fun and motivate players to play longerWhat?
Add a "post-game" progression that gives players more of a reward for sticking it out in the endgame. Mechanics, items, or other that are hard to get and make the game a bit different to play and push players to making bigger bases. For example,- 10,000 Prometheum science packs: Electric trains
- 100,000 Prometheum science packs: Loaders
- 1,000,000 Prometheum science packs: Personal teleporter
- 10,000,000 Prometheum science packs: Linked chests
- 100,000,000 Prometheum science packs: Get stone from asteroids
- 1,000,000,000 Prometheum science packs: Teleporting trains between planets
- 10,000,000,000: Player can land on Shattered Planet (just a bunch of rocks similar to SE's asteroid belts)
- 100,000,000,000: Convert water to fusion fuel