AE added much more to CoH than it took away- unfortunately over time all the creative stuff got smooshed under the farms. Paragon Studios were working on a number of fixes for the search interface and rating system, as well as a map builder, enemy scripting, and player-made cutscene stuff, so we were potentially on track for the AE to make a big, triumphant return.... and then got shut down. Whoops.
As for CO, it's a seed for a good game that's never gotten properly watered or fertilized. It's whole growth has been weird, stunted, and unpredictable. Atari is mostly at fault here, as it forced Cryptic to ignore CO in favor of STO and NW immediately after launch, but Cryptic hasn't managed to do much better under PWE with CO. STO has flourished, NW looks great, but CO... vehicles sounded like a cool addition, but ended up being underwhelming. New content has been significantly lacking in terms of scope and frequency, the Alert system has done worse things in terms of enabling new players accidentally reaching the level cap than CoH's AE farms ever did, and the freeform system lends itself to everyone being tanky DPS or DPSy tanks and basically nothing else. But this stuff is all fixable. If Cryptic had the time and resources, they could just go "okay lets make CO work better, have more diverse builds, put in some content for max level players, etc etc etc", but they don't right now. Neverwinter is a beast that's eaten everything STO hasn't. (though Neverwinter seems pretty good imho) Maybe CO will get some love eventually.
But, regardless of CO's performance or the choices made about the game's content or playstyle, Cryptic just doesn't seem to have the resources to make a FOURTH MMO. Asking them to do so would be crazy. Name another studio that's got three active MMOs, let alone four. Buying up CoH is different, they could stick a small team on that to generate new content(probably similar to how CO's being run- but with the difference being that CoH already has enough content to play and stuff to do at the level cap) and wouldn't have to worry about making that huge initial development team investment like they would with a wholly new title.
It's like a 5% chance that NCsoft will ever sell CoH off though, so uh. I'd just bet on nothing happening rather than Cryptic magically announcing a new superhero MMO.
If anything, they might shift some people from Neverwinter or STO to CO at some point, probably after Neverwinter launches and STO's Romulan faction content is basically finalized. But they won't be making a new game.