One big obstacle Marvel Studios has faced for its entire history involved the complicated rights issues surrounding Hulk (and Namor). While Disney is free to use Hulk in any team up movie, they couldn’t realistically produce a solo Hulk film due to complications surrounding the distribution of the movie.
Despite the common belief on the internet, Marvel could always have made a solo Hulk movie, but the chances of Disney actually distributing it and profiting off the money that went into it were slim. That’s because Universal held the rights of first refusal for any solo Hulk film. They had first dibs at distributing it, and only if they passed would Disney be free to release it themselves. But who would pass up the potential billion-dollar opportunity that comes with a MCU movie these days?
But that obstacle might not be an issue anymore.
I’m only putting this out there as a rumor, not as fact, but I’ve heard from someone very reliable that Marvel has gained full control of Hulk (and Namor) so those pesky distribution hurdles no longer exist. It’s very doubtful they’d ever do a solo Namor movie (due to the success of Aquaman) but they’ll use him as both a villain and potential future Avenger.
This could explain why Kevin Feige invited Mark Ruffalo to pitch ideas for The Hulk. If they could now make and distribute a solo Hulk film on their own, without worrying about Universal, then it’d make a lot of sense to see if there was any ideas out there for such a thing.
Feige did also say that the “chess pieces” were very intentionally left the way they were following the events of Avengers: Endgame, which means there is a future for the big green guy in the MCU. And if this rumor turns out to be true, he could potentially get another solo movie some day.