Just thinking about avenues that haven't been explored yet: are bonus resources usable in some way? The game seems to keep tract of how many you have hooked up in you empire. Like could you have a UB (or maybe a UA since a UB might spiral out of control unless you can give fractional yields) that gave +faith per cattle in the empire for example? (thinking minoans in that particular example)
Olmec. hum. I agree about not having another more cities=lux civ. ideas:
Colossal heads to UI which must be built on (or maybe adjacent? to) stone/marble and gives flat faith and culture per era of existence. Could periodically claim nearby tiles if you want to raid the Inuit script/write that.
Tu and Ku is really interesting imo since you have to choose between annexing the cities or keeping the puppet bonuses (when you do annex no unhappiness from

since they are technically your citizens apparently). Yeah all I can say is that the +1 to all yield works out quite nicely imo.
Kurgen Tomb... UI with flat +faith and +culture yield every time one of your mounted units dies within *5* tiles? Must be built next to luxuries, can't be built next to each other to prevent having too many? This is pretty similar to the colossal heads idea mind you.
I quite like BttLsht's idea of UB that requires horses. I'm running parts of the Strategic Buildings mod, and I really enjoy the extra dimension this gives strategics, so yeah: +1 for replaces circus and requires no horses in the city. cost 1 horse. +2 faith.
Though if you are keeping Outer Pastures then obv this is 2 UB and that's not great.
Can you make a UI require horses?
Re:Re:Vegvar: can't be built next to each other and up the yield. Say +2 culture on railroad (aka pretty castle tech)? Maybe +culture next to river instead of +food to give a boost to tourism late game. They might not even need changing at all but that just seems neater to me, and they can still hit hard without worrying about being OP.
Painted Monastery: Cool stuff. Button looks good in that one screenshot.
Is the whole of Romania getting redone? UA was quite flavourful, though not very interesting.