So how much are you willing to work with? Here are a few options at decreasing involvement levels; I have multiple classes at each one except the last, so if you pick a category I'll pay more attention to that.
Sage is kind of a warlock-y type, which would require checking out a (quite large) new subsystem with lots of new material. I'm guessing this is a no.
Astronomer is a spellcaster-y type, for which you'd need to vet the class, the constellations, the phenomena, and probably the feats.
Dragonmage is a sorcerer analogue, and actually uses published spells so you wouldn't have to vet those, but would require vetting the various ideals (and there are a lot of them). Similarly,
Warcrafter is a martial adept but with 4 new disciplines.
Nullblade has a new mechanic, but doesn't require vetting anything besides the class (and maybe a few feats) because it's all built in.
Shadowcaster is just a fix of the ToM one, with slightly altered mechanics and no changes to the mysteries themselves.
Faux edit: I try very hard to balance my stuff, usually around tier 3 (though the ones that use published spells are higher tier since spells can break the game). No diplomancing from me
I don't really have a single standalone discipline... I would say Essence of Gaia, except that there's no published ToB class which has an animal companion (hm, maybe a Wild Cohort would work...). If that's where you want to put the involvement at, I can consider that.
I ask about homebrew because I've done so much of it that the published stuff is pretty boring to me anymore. Obviously not everyone feels the same.