In my vault, only 1/3 of dwellers are working to produce some resources or Radio Studio and rest 2/3 are just standing idle in the living room or storage room or on endless coffee break. Alternatively, I let them work in extra power generator room that my vault don't need, only just to give them room to stay and keep them employed and happy.
So how about introducing new entertainment room like theatre where these left-over idle dwellers will spend time and have happy life.
The new room type produces nothing and does not expand population or storage limit. Instead, dwellers there will be happier regardless of their SPECIAL. The room cost less than living/storage room and consumes less power per dwellers it hold. Each cell will hold 4 dwellers, so that it consumes half power per each dwellers. 3 cell room hold 12 dwellers.
Dwellers also need room to take a rest. What do you think about that?