My main is a Blaster named Plyx ,he has an evil doppelganger Corrupter named xylP.
I thought how fun it would to "tag" one as the other's arch-nemesis. The Arch would randomly spawn in others missions to fight. But would leave before being completely defeated.
Could make some mechanic or situation where the Arch can be defeated.
Think of the badges....
you have stopped your arch-nemesis X amount of times,
you have never been stopped by your arch-nemesis,
you have defeated your arch-nemesis
etc.