A spidertron that chases you with some intimidating feature would be AWESOME. I can imagine it like a sapper from the game orcs must die 2, which just chase you down and blow up. That would give a very OMG RUN PLS DUN DIE feel to later rounds
Maybe a Spidertron Facehugger, if it catches you, you wil die, and from your body will emerge a different variant of Spidertron! I think MOST of the people here will understand that reference...
I like the idea of the sapper