Well, anything that doesn't involve White people is extremely underused.
Black to Asian, Asian to Latino, Indian to Japanese etc. Not that I'd like to see any of those in particular, but it will certainly be interesting to read.
Also I do agree that race change in general is extremely underused and I could never really understand why other than self-censorship. For me it is one of the most powerful tranformation plots and I can't get enough of it. Especially when the changes are gradual and not merely cosmetic. I also like when race changes are manifested in cultural changes (i.e. religion change, acquired accent/speaking a foreign language, different behavior and so on). That aspect of the transformation I actually find a lot more interesting that the actual physical change. I also don't like instant or magic change as well as mind control. It has to be relatively "realistic"... Just my two cents.