As a 4* player (about 30 champed, no 5* above lvl 330) I regularly find that the only time I ever use my 2* is in the required node for PVE or in Deadpool nodes. My 3s get some play in PVE and as the required character in PVP, but the 2s just sit around.
My suggestion (and I can’t take full credit since I’ve seen it mentioned before) is to incorporate the Combined Arms PVP into a season-long PVP event like Shield Sim.
This wouldn’t have to replace Shield Sim, but they could sit side-by-side, with progression-only rewards. To prevent score blow-outs, instead of the devs raising the bar on season progression rewards, they could set the cap for both Sims at say 1500 points.
And I would finally get to see my 2* Hawkeye work with M4rvel for more than a few days each month!