I do not know what to put after Tier Three, but I think that there should be something. The whole plot is founded upon the premise of getting a power source from the core. There could be a few (but not limited to) a few option:
1. The power source takes you home. (you get an achievement)
2. Something else goes wrong with the ship, and now you have to search the surface of the planet.
4. Some obstacle prevents off-planet escape. (maybe a moon, or a blockade)
5. The power source is not what we think it is. (i.e. it sentient or something sci-fi)
6. ???
The point is, I would like to see this game expand a bit, resolve the plot, and become more of a sandbox. (Even different havens could become different planets...) This all being said, I would like to see this eventually but realize some more PvP should come first.
its not like this hasn't popped up time and time again
it would neat for SK to have new planets with new and unique sets of levels/monsters and have the option to return to cradle but we wont know until they release the core (if ever)