Could it be another Avengers cartoon? Doesn't season 3 of Assemble have some new creative heads on it? They might be preparing to bring it to a close and try something else.
AA's goal of emulating the movies has kind of become superfluous since the Avengers in the MCU no longer resemble the TV incarnation. Trying something original might be a smarter bet in the future.
Their other upcoming movies, Black Panther, Captain Marvel, Doctor Strange and the Inhumans, all currently appear in other shows anyway.
I'm pretty sure season 3, Ultron Revolution, is the first season that Wacker has been able to have major influence over while the story editors are essentially still the same. I doubt they'd try and do a whole different Avengers cartoon, or even a Spider-Man cartoon, when they seem to be content just rebranding the shows to be closer to the movies or the new directions they're going with.
True, we are going into season 3 with the same basic team as season 1-2, but we're at least getting one of the New Avengers with Vision, while you've already elaborated on why Scarlet Witch probably won't be popping up, and Rhodey's non-existent which probably means no War Machine

.
We also have Black Panther coming up, and another Civil War, so season 3 seems like it'll be bridging the gap between Phase 2 and Phase 3 elements into the cartoons

.
But yeah, it would be nice to see Marvel Animation go in a more original direction for their cartoons instead of just wholesale following and referencing the movies

.
I've always liked the idea of a Marvel Super Heroes not tied to one specific brand or property. They could throw in Wolverine, Thing, Squirrel Girl, Silver Surfer or whoever else they wanted, and use any villains they wanted and Marvel would still be the name up front instead of X-Men or FF or any other property they don't own the film rights to. The series' ideas they go with usually turn out to be the last thing I think they should go with so it would probably end up being something like an Ant-Man series.
The idea of taking the only characters Marvel Animation uses from the franchises that Marvel won't promote in animation because of the movie rights, and putting them into a series together along with Squirrel Girl where they just fight random villains, actually sounds like a great idea

.
Ant-Man is the only major movie property they have out right now, and while it was a lot more successful than people expected, I don't think it was popular enough to warrant a solo Ant-Man cartoon. It definitely wasn't Guardians successful

.
Though an Ant-Man cartoon would be interesting if only because then we'd likely see Pym in animation, and some incarnation of Wasp. But, as much as I like him on Assemble, I don't think Scott's a compelling enough character to hold his own series, so I don't expect we'll be seeing him get the chance anytime soon.