I'D love a Captain Carter movie! Peggy has always been one of my favorite MCU characters, and the What If...? appearance strikes the right balance of brains, beauty, brawn, humor, and empathy. I'm guessing the question would be if Chris Evans would want back in, especially as it would likely be a depowered Steve Rogers he'd be portraying. I understand it was a painstaking process to bring that version of the character to the screen in "The First Avenger".
Regarding T'Challa, I respect their decision as creators to retire the character, but I believe this is a mistake. While I understand the gesture is an homage to Chad, a character should transcend his/her actors. Where would Sherlock Holmes be if the character was retired with the very popular Basil Rathbone? We'd have lost dynamic performances and interesting interpretations by Christopher Plummer, Christopher Lee, Peter Cushing, Jeremy Brett, Ian McKellan, Johnny Lee Miller, and MCU alumni Robert Downy, Jr. and Benedict Cumberbatch. I've not exhausted the list, either. We've had at least two major recastings in the MCU so far (Hulk/Banner, War Machine/Rhodey), I believe the character of T'Challa could have been a third, and still have been tastefully and respectfully done.