It would be nice to be able to level all adventure schools from just killing mobs. A way to make this possible would be to not let your rating limit your XP. I understand as the game is now that your high rating on a character makes you really strong and many of the mobs miss you.
We should still be able to master abilities to use on our character. It's a good reason to do several, but take the rating of our characters out of the equation. This way we could easily swap to a different school that we may want to try and be able to level it just like the first one we did. Kill the mobs at your level not your rating level for xp.
I'm betting this would upset some of the community but, I think it would allow the game to not be such a grind. I see people always asking for DXP, this is because the game is hard to level the adventure side of the house. The first 4-5 100's aren't too bad, but after that it is. The Noxious maggots in doom made it easier when those were (not per design)? Players were forced to do what they could to level.
I tried to think of a downside to what would happen if your rating didn't effect how a mob sees you but, I really don't see one. It would make the game a little hard in some aspects but, you would see a lot more different classes running around I think. As it is now, I think players think long and hard about what schools to do in fear of not wanting a really high rating then not being able to level the core classes that make a great character like Warrior, Mage, Healer, Spiritist, Druid!
-Kor