Hum... What about something like... related, non-related and opposite guilds...

First though, you could only train spells and skills lower than.. level 6 or so. I don't think I'd ever want to see an AM with ithian, oblivion or iron will. \:\)

For say AM, related guilds would be... enchanter and elementalist. Training in these guilds would cost 2x. And you would need to be level 25 or so before you could train anything in those guilds. Spells would be limited to *4 or so, skills to 50%.

Neutral guilds would cost 4x, and need higher level, say 35 before you could train. Spells limited to *2, and skills to 25%

Opposite guilds which for AMs would be fighter and thief I guess, which you can't train in at all.

This is very basic and with lots and lots of problems. For example, for enchanters AMs would be a related guild. But AMs have some very unbalanced low level spells, like Point Defense (level 2!), which... would be very nice for enchanters and elementalists to have, to the point where you would be insane not to train it. \:\)

Another obvious problem is the point where guilds have nothing more to train. A paladin needs level 36 magic or so to reach the point where they have every spell in the guild at *6. So they could just focus on crosstraining from there. Enchantenters aren't even finished at level 45 magic, really, and get considerably less XP.

So... lots of balance issues to resolve before even thinking of this. \:\)

This is just vague ideas keep in mind \:\)