Lang would be absurd as an MCU
Avenger. Didn't say anything about his Avenger-ness legitimacy elsewhere.
You can't inundate the silver screen with the same character density as a comic book (multi)verse. Therefore I feel the priority is to have mostly classic Avengers. It's worrying that we already have the lame Falcon coming in. Why would we want to make it even further extra-extra-lame by throwing Lang in there, too? Does not compute AFAIC.
My worry is that after the "To Steal an Ant-Man" arc is complete, who cares about Scott Lang? You can't have one-off plot device characters (which I'm suspecting the Falcon and Lang to be) and then make them Avengers. Granted, not all Avengers are/will be heavy hitters, but even the "minor" ones should come with some sort of built-in audience affinity towards them. I just think it's beyond the pale that we might not get all of the core classic group together on the silver screen with the current crop of actors.