If Joss Whedon loved her so much..
He could've given her the perfect role in one of the Avengers films tailor made for her: Janet Van Dyne aka The Wasp.
'When there's no more room in Hollywood, remakes shall walk the Earth.'
Buffy (his most successful series, outside of Agents of SHIELD, the success of which is propelled entirely by fans of the Marvel franchise and not Joss's name) was never "big". It only ever had a few million viewers consistently and usually hovered near the bottom of the Nielsen primetime ratings ranking. And of course we know how Dollhouse and Serenity wound up.
And being a writer on a big movie doesn't really mean anything in terms of casting. Heck, even directors have to bow to the studio when it comes to casting decisions. Even Spielberg or Michael Bay couldn't cast some nobody in the lead of a big blockbuster movie. Studio execs are going to insist on recognizable name so they can put it on the poster and get butts in seats.
Bottom line: Nobody at Marvel is just (or would have just, since Whedon isn't really associating with them any longer) going to let Joss cast a moderately famous television actor in a tentpole franchise film.