Idea 1)
Why is there no more buildings to build after 100 population? I think a great idea is to create the game function that new resources, tools, or actions. Bringing in purpose for new room types and variety.
Idea 2)
Maybe you can design an update that allows you to purchase with caps more buildable earth other than thhhe straight down variety. Buing more space would then require higher populations, more resources for different things such as maybe a fire station type room, something that sends dwellers throughout the vault souly to extinguish or vanquish any incident within the vault.
And to make the above idea one step further, create new styles or variety of incidences opening up to more emergency style rooms. Even a military outpost souly for defence against raiders and deathclaws.
Idea 3)
Raise the level cap on dwellers, and SPECIAL cap. Further advancements would require stronger dwellers to properly defend and survive.
*all 3 ideas run together you see? It can be done and would make this beautiful game so much better.*
Hope to read some ideas from players and developers