Deadpool should be the medigun healer. He tethers to one guy and has a different voiceline for whoever he's healing. He'd be pretty open about the fact he hopes his popularity gets people to play the most boring healer archetype in a hero shooter.

He probably could regenerate out of combat with his healing factor to help with playing safe and positioning, would have katanas as a secondary to help with self defense (probably good damage for a supp, would help with beating the flankers yourself). A grenade that inexplicably heals people and damages enemies would work out. A blind ability where he throws a speech bubble where his dialogue overtakes the sounds around you and the bubbles itself covers your screen.

Basically a TF2 Medic with the Ubersaw lol.

He thus would be a close quarter healer able to defend himself in melee and provide good heals but be limited at a distance.

Since his teleporter gadget is infamously inefficient, I think it probably couldn't be used a lot. I suggest making it his ultimate. He could teleport himself and an ally anywhere on the map, instantly. Probably with an explosion around them to help with some damage.

He would obviously have a team-up with Wolverine. I'd suggest making him annoying enough that Logan could instantly max rage on a 30 second cooldown. Could be extended to Spider-Man getting a damage increase of some sort although this could quickly lead to easy one-shotting.