Hmm, very curious who this mystery combatant will be!
I'm gonna make a guess list, if allowed.
Firstly, since this tournament is more pre-BeastMachines show-based, I'll list obscure characters from the show.
-CyberBee
-Starscream
-The Vok!
-Snow Stalker
-The Tripedicus council of red guys
-G1 Megatron from the disk (badly voiced, but not as bad as Stascream)
-Transmutate
You could also go with classic BW toy characters, and really surprise us with the McBeastWars toys!
Panther
Rhino
Beetle (known in my family as The Geo-Spider!
)
Manta Ray
Under-3 (
)