Those characters that die in dark of the moon, die really violently, their deaths feel so permanent, for me it would seem strange seeing them in a sequel. If they were just shot down, or had the crap beat out of them, and they just fell into a heap of parts, I'd be ok with seeing the well known characters return from the dead. But seriously these guys were executed with fatalities Mortal Kombat style!
In a sequel I'd like to see lots of new characters, If not I see failure. A new villian would honestly be more exciting.