I think they're blatantly gonna just come on about level 10 with the 'runner zombies' and add a new element to the game...
I think they're going to be much faster than the zombies, like you cant out run them, but I think they'll also be much weaker... Prolly do same amount of damage, i.e 2 hits bring you down...
I also think that seeing as they are on fire when you kill them, they'll leave a smoking corpse like when you kill a zombie with the flamethrower, it blinds everyone and annoys most people (lol, personally my fav gun

).
Also, im saving this post incase im right and I prove all you wrong! Ha.