I honestly didn't mind Black Panther, I was just of the opinion it wasn't the greatest superhero movie of all time that many were floating the idea of. Still think Wakanda vs. Atlantis for the sequel would be awesome, cool to see Earth's two most advanced societies go at it.
Think you are half right they likely want to right Renner out but think they are looking to establish the young avengers.
Hawkeye's daughter Kate Bishop has already been introduced and she's a young avengers member. Same as Cassie Lang who's been introduced now as well (and I bet will get a similar show after Ant-Man 3). Definitely need to introduce some younger dudes to have on the team though as few younger male characters to take up the mantle and I doubt they will include Blade or Moon Knight as I think they'd be too old and gritty. Means we might get Kang as the next big bad though.
Wouldn't be surprised if they even add She-Hulk to it since she's getting her own show and they could cast her on the young side at around 25.