I mean no disrespect in saying this - if you are not rocking OBW in 2* land, you are not doing it right.
She is amazingly useful and fits in with any 2* team that can be put together AND a lot of 3* teams. Sure, hit points are low, but that 
AP steal is one of the best active purple skills in the entire game and her 
that increases countdown timers (and heals!) is HUGE in PVE. Put her on a team where she tanks 
, 
, and/or 
and she double dips on damage and steals AP in that color.
TL;DR - OBW is a BEAST.
OTHER MUST HAVE 2 CHARACTERS*
- Ares - Dude is a damage machine. His 
is absolutely huge, 
, while self-damaging, is a big damage dealer, and 
, while weak can chip off some damage to the entire opposing team. Couple this with him scaling brilliantly and he is a must have.
- Magneto - He generates 
& 
like a champion and his 
skill is great at his rarity level. 
is kind of meaningless, but will get some board shake going.
- Storm - The essential teammate for Magneto. He generates the 
, she drops the team damage and a great stun to the character up front. Her green is great for board shake and collecting AP, and her yellow may not seem like much, but over the course of the match it can mean the difference between winning and losing. Never mind her low hit points, she is completely essential as a 2* player.
TL;DR - OBW, Ares, Magneto & Storm will be all that anyone really needs after moving on to the next stage. They should be optimally covered and max leveled on everyone’s rosters.
Characters to keep until getting their 3 versions covered - Captain America, Daken & Thor.
*Characters to consider using - Human Torch, Wolverine, Hawkeye (for Speed Shot alone, he teams well with Magneto & Storm), Bullseye
*Not needed/YMMV - Captain Marvel, Moonstone
*Super-Secret Line Club Awesomeness - Bagman: If you have a fully covered, max leveled Bagman on your roster you are a legend. A legend for taking the time and wasting the resources on a completely meaningless character. And the masses will bow before your greatness.
I’m a fan of holding onto whoever you can, because of 2* characters being essential in PVE and getting boosted in both PVP & PVE. They can save you some health packs when working with easy nodes or taking on the early matches in PVP. But, roster slots are hard to come by and the most important part of the game, so it is inevitable some hard decisions will have to be made by a lot of people.