Once the dust from Endgame has settled, Thanos is surely done with the MCU, one way or another. He managed to unite the entire force of Marvel’s heroes against him and still come through Infinity War fighting; he leaves a massive void for the next villain to fill.
Thanos has been lurking in the background ever since Avengers brought a curtain down on Phase 1, so Marvel might not want to rush their next one in immediately. The MCU has patiently built their world and linked their characters together, so don’t expect a ‘new Thanos’ right away.
However, plans will be in place to have someone pulling the strings behind the scenes. It may be another mighty foe like the Mad Titan, needing the Avengers, Guardians and everyone else to team up, or it could be something altogether different.
A more intellectual, technological or mystical villain could keep the heroes on their toes and allow different Avengers to shine.
Whoever it is, Marvel certainly has a very deep bench when it comes to bad guys and gals. Even if all the ones listed here can’t be the next big big bad, they can still challenge the heroes in solo movies.