Oooh, i like the idea of sub-classes! It'd be like Seiken Densetsu 3, one of my all time favourite games O_O
You could have the classes relative to the classes you chose at the start, and you could go to your chosen god in order to change class.
So like, you choose a starting class of...priest, say.
You level up until class-changing level, then choose a god you want to get a class from. If you went to say Govannon or something, you'd change into a Monk class (as in fighting kung-fu funky monks
) and if you went to say brigid, you'd be made like a bishop or something (which would have advanced healing'n'regeneration'n'healing prayers'n'stuff).
Or an alternative, which would be very complex indeed, would be to have different sub-classes from each god, and those came from the start classes.
So it'd be
-priest-
-> Worship govannon
-----> Fighting Monk
-----> Paladin
-----> Some other spoony class
-> Worship Bile
-----> Plague Bearer
-----> Necromancer
-----> Some other spoony class (lol, as you can see, it'd be hard to think up lots of different classes)
etc etc etc, anyway, you get the idea