I think it would be cool for a new Civ to have a unique Amphitheater which had 3 slots for GWoW

[Leathaface]

Instead of the usual 2 slots for Great Works of Writing. Every 2 amphitheaters this Civ has would be able to hold what 3 amphitheaters could hold with other Civs. Plus maybe the amphitheaters could theme? Usual amphitheaters don't.

Or would that be OP?

 

[Phrozen]

Maybe Assyria if they get in.

 

[Alexander's Hetaroi]

The only unique amphitheater which I have come thought of would be something like an Opera House, probably best reserved for an Italian Civ, or possibly Austria. Instead of adding more great works of writing it would be cool to let it get Great works of music slots as well after learning both the Humanism and Opera and Ballet civic.
The Byzantines are possible as well, but it would make more sense for them to get something else.

 

[AmazonQueen]

I think it could work and wouldn't be unbalanced if its a major part of a civ, The question is who? Maybe Celts? Eisteddfod are held every year in Wales as a celebration of music and poetry so rather than theming make it able to hold music and writing.
The biggest weakness with this idea is that Eisteddfod are events rather than places.