How about this... Instead of crosstraining, why not a free-for-all adventurers guild It would have all the skills and spells you can cross train? Something like, like.... level * 3-5 trainings.... That way it'd be easy to regulate that no silly powerful spells/skills that need to be guild unique gets crosstrained, and it could add a nice deal of variety. It'd be even more fun if there was a casting skill and a few simple spells for each god. (of the gods that don't already have a guild of their own).

Something like that... Idea not fleshed out very fully. \:\)