Barracks: +2 science down to +1 science
Aqueduct: 15% food carried over down to 10% food
Baths: +10% culture in city during GA down to +7%; 1 gold maintenance up to 2
Armory: +2 science down to +1
Chancery: 2 gold maintenance up to 3 gold maintenance
Stable: +3 production down to +2 production
University: 2 gold maintenance to 3 gold maintenance
Constabulary: 3 gold maintenance up to 4 gold maintenance
Grocer: +3 food down to +2 food
Opera House: +10% culture down to +7% culture; 3 gold maintenance up to 4
Windmill: +2 food to Grocers and Granaries down to +1 food
Factory: +2 gold to engineeer specialists and +2 production to merchant specialists down to +1 each; +3 production down to +2 production
Hotel: 4 gold maintenance up to 5
Military Academy: 4 gold maintenance up to 5
Museum: 4 gold maintenance up to 5
Public School: 4 gold maintenance up to 5; +3 science per turn down to +2
Train Station: +25% production down to +20% production
Zoo: 4 gold maintenance up to 5 gold maintenance
Broadcast Tower: +1 culture and +1 tourism per 5 citizens down to per 4 citizens each; 6 gold maintenance up to 7 gold maintenance
Coal Plant: 3 coal produced down to 2 coal produced
Hospital: 5 gold maintenance up to 6 gold maintenance
Research Lab: +4 science from Hospitals, Factories and Medical labs down to +3; +33% increased Great Scientists rate down to +25%; 6 gold maintenance up to 7 gold maintenance
Stock Exchange: +3 gold per turn down to +1 (or add 2 maintenance)
I don't really know enough about the later eras to comment (and G has commented that later eras buildings need to be a bit OP to generate a return on investment quickly).
I lowered the main yields of the plainly overpowered buildings (notably all the science and culture buildings except for the first two eras when yields are more sensitive), and for the rest I either increased the gold maintenance or lowered the yields slightly if it didn't make the building bland. I didn't touch the defense buildings (castles, arsenals, etc.) because these buildings are too important to be crippled with more maintenance in case you are doing badly.
I increased the maintenance for buildings that weren't too strong but niche, so that you had an incentive not to build them everywhere - zoos and hotels, for example.
In the end this is very premilinary, just a sketch; perhaps we want more general changes (+1 maintenance to every building of every era; -1 science to everything that gives science, etc.) too. This is more to spark discussion.
(I used this wiki page:
http://civ-5-cbp.wikia.com/wiki/Buildings though be wary that not everything it states is up-to-date.