I think Oda will stick to asian themed mythical zoans, or perhaps more specifically japenese, the fact he gave Catarina Devon one.
Daibutsu (Buddha statue), Phoenix (yes there's a chinese/japanese variation, looks similar to Marco except color), Onyudo, Seiryu, 9 Tail Fox, whatever the frick Yamato's zoan name is.
Only exception is Luffy's, he's an in-verse mythical zoan. So for any other future mythical zoans people might refer to the japanese mythology for now, till we get an exception
Would have been dope for some norse mythical zoans