How about settlements. Why should I play them more then once. I found them annoying.
Each of the settlements has a main reward building which is quite good.
Vikings - Yggdrasil lvl 9
Japan - Shinto Temple lvl 9
Egypt - Royal Bathhouse lvl 6
Aztecs - Sun Temple lvl 9
Mughal - Mughal Temple lvl 6
Some of the secondary reward buildings are good too.
Also, if you build the Flying Island GB, you can get quite a few 1-ups from your settlement (plus fps and goods).
But the main reason I am persevering with the settlements is I am a completionist and it is a goal that keeps me interested in the game.
p.s. I only play on one world too. Trying to manage multiple worlds just seems like overkill. And I feel the diamond farms are a bit of a rip-off.