What about Spider Woman? I think that Marvel has full rights access to the character; if I remember correctly it is because Spider Woman wasn't created as a subset character under the Spider-Man banner but rather an independent character (with a primary goal of locking down the trademark).
And if we're focusing on younger heroes, there is one in particular that needs mentioning --
.

A Squirrel Girl movie with the tone like that of
Guardians of the Galaxy could really take off.