What kinds of mechanics have already been used for skillmonkey type? Usually it's just +xd6 precision damage somehow, but there are a few classes that have something more. Factotum (Inspiration), Ninja (Ki), um, any others? I think the Malefactor is a skillful type, although I don't really know of that many homebrew skillmonkeys. Kellus's Truenamer came close-ish, I think.
Other than spellcasting and ToB, I don't think there are any subsystem around with the breadth to support so many ideals. Even ToB can only do it due to the amazing homebrew support it has. What was it, 160 or so homebrew disciplines? On top of the 9 that were actually printed in ToB? Spellcasting might be big enough because it gets all the support WotC has to offer, but it isn't neatly sorted by theme like ToB's disciplines are. Domains are the closest you'll get to themed spell groupings, but they're a little small and there are too few of them to use exclusively. Offhand, I'd guess there to be about 75 domains, plus another 20 or so Wizard domains in UA. That might be just enough, although since they don't all sync exactly with the dragon types that are around, you might have to make a few more to fill the gaps if you go with domains. There are, what, about 50-60 ideals planned?
What might be interesting is to have a combination of subsystems (assuming you can't find one that's big enough already). Depending on the ideal, you might get some truenaming, incarnum, binding, ritual casting, inspiration, spellshaping, synthevolving, morphing, implementing, slicing, activating, echocalling, chronomancy, etc. abilities for the day. There are enough homebrew systems around, you can probably do it with no more than two or three ideals granting the same system, although you might want more overlap for the larger systems so people can get access to more than the bare minimum of it.