I'd like Lavender to be in his Harem. She gets too little love as it is. If she's an airhead in canon, you could just say it is an act, to keep potential enemies from taking her seriously. As for the "take strong witches for moar power argument", power levels can be adjusted from canon and maybe it really should not be intrinsic. In other words, anyone with enough practice can take out Voldemort or Dumbledore.
Ginny should be slave at best. I'd prefer you kill her off, however. Perhaps as an experiment on how long must one apply cruciatus to kill a person. I can't put enough of an emphasis on this: no Ginny for the harem.
Enslave Cho, a hot asian chick is needed. It might as well be her. Su Li too, two hot asian chicks are better than one.
Hermione can serve as a avada kedavra shield for Harry.
If you go for a professor, I'd prefer Sinistra.
Just kill off Bellatrix. Extended torture, avada kedavra, collateral damage with area of effect spells etc. Anything is fine.