Would love to see full on zombied Borg intent on bringing everyone and everything into their collective.
I'm sorry but the Borg would be awesome. The masses love zombies, the masses bring in money, we get entertained, win-win.
How about another bad guy who migh not be bad? An anti-hero of sorts who has a good heart but must do necessary evil to save those he loves even if it is at odds with Federation values. No revenge, just trying to do what he or she feels is right but pissing everyone off in the process. Like the guy who detonated Section 31 in STID.