Having some spellcheck issues there, Plane?
Never happen. And probably shouldn't happen. Audiences will want new stuff. If nothing else, they'll want to keep seeing superhero-versus-superhero action. Can't have Iron Man, Thor, and Cap mixing it up again, can we?
Since characters can be introduced in Marvel films leading into the Avengers (as you noted) then the concerns you have are largely addressed.
So audiences want new stuff, but need to see the same old stuff (superhero vs. superhero)? Which is it?
And no, intros in other films don't address it. If the Avenger roster doubles, you can't provide a balance of story/screen time to each character. Avengers already clocks in at 2.5 hours. Additionally, if I'm using solo-hero films to intro or develop characters just to avoid future Avenger plot-time, I still have the problem of # of contractual appearances. Also, you've got the budget of a single Avenger film to consider. If Ant Man also showcases an actor for Wasp, Vision, Ultron, etc. I'm adding to the budget of future Avengers installments.
Avengers is a runaway success. While I'd love to see Avengers 2 & 3 have equal success, the odss are against it being of the same magnitude.
For every fan who says "add my personal favorite Avenger", someone else's favorite is getting short shrift or reduced time. And there are a TON of Avengers.
They'd actually be better off doing spin-off Avenger teams ala West Coast Avengers/JL International where they could showcase more characters without mucking with the core team.
Also, my gut tells me that if we see any new Avengers, they'll be trying to diversify the team. My money would be on Scarlet Witch, Ms. Marvel, or Wasp, Falcon, and/or Luke Cage. But yeah, I'd love to see the Vision as well. However, I realize all the plot baggage that entails and think it would be better served in Ant-Man movies.