I think dividing up the Dwarven magic system would work well; let them keep earth-magic and runes, but pass off the other elements to otherwise magicless races with warlocks being the only group that does everything. Succubi are already associated with fire, kobolds and air seem thematic (I'm imagining ninjalike 'bolds using stealth and disorientation spells), and water will probably make the most sense around the fishpeople/nagas. Humans could probably have the alchemy system with a little tweaking (since alchemy is the least magical of the MWDF magics, and humans are the least magical of the MWDF races), maybe as a slightly cheaper alternative to purchasing power if you happen to have a lot of "reagents" on site like brimstone, mercury, rare gems, magic plants, etc. Gnomes and elves would probably share "nature" magic with gnomes emphasizing friendship with animals and elves emphasizing friendship with plants. "Good", and "Evil" magic could probably work as a sort of commonwealth that everyone has access to- though for flavor the "crueler" races could be limited to only basic "good" magic, and the "civilized" limited to basic "evil" spells.
(EDIT: Forgot about succubi, *_*; ) Goblin magic can be a sort of "WarlocksLite" with minor spells for anything but lacking skill or power at everything but "evil" spells in which their "natural evilness" allows them to excel.
(EDIT2: Forgot the orcs too!) Orc magic seems like it would be primarily blood-focused, since everything they do is centered on the blood, skulls, scalps, and souls of their defeated foes with a variety of effects centered on killing more foes. In addition, AFAIK there's no way to train an orc to be a dreamwalker, it's something they're apparently born with.