THe allomancy system is coming along nicely, at least with the basic 8 metals. Perhaps gnomes for allomancy? Or perhaps it's a freak mutation among the urgals? Another possibility is that it arose not from one race, but from the union of one or more. Maybe a half-urgal dwarf married a half-elf and the kid was a mistborn? Just tossing out thoughts, though that last one makes me giggle.
I like psionics myself, but I already put in my two copper, so I'll shut my yap about it now.
Incarnum is interesting, and no one ever said that you can't cut out the other classes that don't work (even if it only really leaves totemist). Perhaps it would be better if you limit it like this for your world. It gives the urgals a unique culture to bring to the table, a power that no other group is 'primitive' enough to consider.
I don't know much about the ToB and ToM stuff, and even less that's helpful. I do enjoy the shadow magic stuff from ToM, though.
They're 3rd edition, but I suggest looking at the Encyclopaedia Arcane. Star Magic is particularly interesting.