I personally think that out of the roster we can pull from, convincing Valve to give us Chell (of Portal fame) to be DLC, and Abes rival if SuperBot does indeed give us Abe.
Think about it, Chell could have Potato Glados on her portal gun, who tells Chell that a passing by Abe is a unique creature who would make an excellent test subject. Abe, remembering his Cola rescue days, panics and attacks before Chell can explain that she doesn't agree with Glados (Also keeping our silent test subject from talking in the process.)
Furthermore, thinking on this, I made up a fictional movelist for Chell, and was curious to know what you all thought.
Neutural Square - A downward swipe with the Portal Gun Side Square - A kick, and maybe a couple of Portal Gun bashes with multiple presses for her basic combo. Up Square - A hit directly above her with Portal Gun Down Square - Creates Acceleration Gel that trips anyone a foot or two on either side of her, netting a small amount of AP, but causing a back crumple (Like FPs air soldier) Air Neutural - A quick kick. Air Side - Several scissor (sp?) kicks. Can continue to kick until she lands. Small AP gain per kick. Air Up - Wide arm swipe above her head. Air Down - Creates a red portal trap on the ground. Anyone caught in the trap gets stuck for 1.5 seconds when the portal encloses around their body mid-fall. In done on a platform, the trapped character falls through the platform onto the ground and crumples on impact.
Triangle Moves Neutural Triangle - Chell ducks behind a Weighted Cube, blocking any 3 hits before the item isdestroyed. Unlike Drake, Chell can not attack while behind the Cube. Side Triangle - Chell throws a Memory Core in an arc forward. It blows up upon impact with a hard surface. Up Triangle - Throws Potato Glados into the air directly above Chells head. Anyone hit gets an electric shock. Down Triangle - Creates a Laser Cube that stays on the field. Can be destroyed by 1 -psyichal attack. Laser Cube will reflect ANY projectiles, bouncing them in a random direction. Air Neutural - Weighted Cube appears in front of Chell, but drops ontothe head of any unsuspecting foe for damage. Air Side -Chell kicks a memory core forward, and it explodes on impact. Air Up Triangle - Chells gun gets an electric shock from Glados, and anyone touching Chell in those breif moments get stunned. Air Down Triangle - Air charge diagnally, Portal Gun extended first. Similar to Raiden.
Circle Moves Neutural Circle - Chell Aims her gun (with the help of a sniper scope) and createsa a portal. Use twice for two different colored portals. Any characters can go through the created orange portal and come out of the blue one, or vice versca. Side Circle - Chell ports up some bounce gel ahead of her, which damage and bounces any opponent touched by it. Up Circle - Chell creates Bounce Gel below her, giving her a bigger hop than simply double-jumping. Down Circle - Chell jumps back, revealing a sideways turret, which explodes and sends opponents flying across screen. Air Neutural - Same as above. Side Air - The bounce gel is shot diagnally downward, same effect. Up Air - Same as above. It is also worth noting that Bounce Gel can be used by anyone. Down Air -Chell jumps off of a sideways turret that explodes on ground impact. Grants a third jump to Chell.
Supers Level 1 - Chell throws a lemon, which creates a huge blast radius, similar to Toro/Jak's 2, but slightly smaller range. Level 2 - P. Body and Atlas appear,shooting portals around the stage and running through them. Anyone hit by either robot dies. Level 3 - Glados takes the reigns, and gets transferred back into her body, and transforms the stage to an Aperature Science testing facility. She shoots rockets and bullets for the duration of the level 3 and they fill the screen much like FPs. Furthermore, Weighted Companion Cubes rain down from random portals appearing over the ceiling. Anyone caught in a rocket explosion, machine gun fire, or a WCC is killed. Short Duration (around FPs duration for her Level 3), but somewhat difficult to just dodge/tech away from.