#2. Conor McGregor - former UFC double-champion

Conor McGregor is the biggest star the UFC and the MMA world in general has ever seen. His movements around the world are tracked by both MMA and mainstream media alike. Should he start up his own promotion, you could be certain it would receive plenty of attention.
Given 'The Notorious' is known for his Irish roots, it would likely make sense that his promotion would take place in Ireland. Should the promotion have success, he could likely expand further into the United Kingdom and attempt to rival the already-popular Cage Warriors.
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With the success McGregor has had, fighters would surely be keen to align themselves with him, as they could become a big name very quickly. With McGregor approaching the end of his contract, there's also a possibility he could be available to fight in the promotion himself one day.
#1. Francis Ngannou - UFC heavyweight champion

It is well documented that Francis Ngannou has no more fights left on his current UFC contract. That gives him a wealth of options and perhaps one of them could be starting up his own promotion.
If Ngannou does not re-sign with the UFC, he will be able to command a more flexible schedule whether that be in boxing or another MMA promotion. That might free him up to find and potentially compete in his own MMA promotion. The most likely location for such a promotion would be in Africa.
Kamaru Usman has already taken some similar steps, but Ngannou's could be less Nigeria-specific and tour all parts of the continent. Additionally, in the scenario where he is no longer under contract, 'The Predator' might be able to be far more involved than Usman is, as he would not have to deal with UFC commitments.
Ngannou overseeing and competing in his own events could finally provide Africa with the big MMA promotion it deserves.