What about reverse engineering? I mean, try to think about comps which can be beaten by using most uncommon characters/tactics..
This is a sort of brainstorming, so be patient, no final solution here and of course feel free to add of your own or to correct me.
So.. What are most common things used by now:
Focus firing
Dispel
Outspeeding
TMM
Dodge
Taunting
Now we've to work something about deny the most of that stuff. My guess is that the best defense, since AI will play it for you, lies in RNG and disrupting typical attack patterns as much as you can, to achieve it i'd say that countering focus firing would be my top priority.
In my opinion Teebo and old Ben are the two leaders that better give you a little bit of both. They have specific counters, like b2 and ventress, or fast stunners but those are not widely used in arena.
Second thing you don't want is to allow an attacker to control battlefield, and most people do that by using dispellers and StHan/RG. To me the best counter is b2 and Daka.
Third thing you don't want, as I think that you can't do without at least one major DPS toons, is it being focused down, so i'd take leia for permastealth (I know AI many times doesn't recast stealth soon enough, but Teebo could help with that) or Rey for foresight. Both are fast, hit hard, and have a way to self protect.
So in the end i'd try this:
Teebo L for focus firing disruption, dispel and TM shenanigans
EE to revive/healing if something goes South and debuff cleaning. He has some TMM too if omega on basic
Daka for stun fest (omega on basic) and backup healing/revive
B2 to deny control through dispel and buff immunity
Leia/Rey to deal some damage.
Other options are to sub in Old Ben for Daka and/or, for those who can, sun fac for b2.
The only FTP option to avoid b2/sun fac is RG, even if he does half the stuff the other two can do, he'd still be a good addition to the party!
I didn't consider current bugs that some toons has, hoping for a fix coming soon™
