Vampires, ghouls, zombies, thralls, demons...
It could actually be quite interesting. If they only operated at night, you would probably need more bases with Skyrangers so you had any chance to catch them. Good setting for veterans who want more challenge and different threat.
It would probably need some coding done, but it does not sound too hard. Bigger part would be making the graphics and stats.
And yeah, it would make sense if some alien races did this too in X-Com
boolean vampire = true;
if ( daytime )
alienmission.abort();