So would it only allow support from any god, or would it accumulate favor from all gods?Just another idea for statue and/or a new wonder: Turn it into a "divine shrine" devoted to all the gods. This allows you to support a city with any mythical units, regardless of the god worshiped in the city.
Also, if you are looking to add a third wonder in the game, it can be worked into the city on the right of the market place.
I think it would eliminate some of the strategy associated with picking gods for cities. Either balancing cities, and picking gods based on proximity to front lines.
Slightly off topic, but there should be more benefit for spells to cities of the same god/less effect for spells to cities favoring other gods. So, Kingly Gift to a city that also worships Poseidon should provide more wood than to a city worshiping Hades. Similarly, Earthquake might have less effect (what god wants to punish their worshipers?)