Best solution.
After u beat the fortress, just open a teleport to another world (this world only have players that beat the fortress)
All players can choose the ammount of bases to teleport and have 1 week to do so.
They go with all resources they choose. After that, they can only teleport to another world if they they beat the fortress again.
The credits would work for both worlds... But not the rest of resources.
If you go with few bases, you get weak in the new world that only have experienced players.
If you go with all, the enemies will go after you...