#2. Conor McGregor vs Donald 'Cowboy' Cerrone

With regards to Conor McGregor, it must be said that all of this is under the assumption that he will return. Given that he's made more money than he knows what to do with and that his motivation clearly isn't the same, one can't blame him for not returning.
But if he does find that hunger to return, there's nobody who can bring the passion back in him the way Donald 'Cowboy' Cerrone can. Cerrone is a true fighter and was McGregor's originally-discussed return opponent.
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It didn't work out due to issues with management, but now that UFC 244 is a title-less main event (if you discount the BMF title), then they can easily do a Conor-Cowboy PPV main event in late 2019 or early 2020.
#1. Khabib Nurmagomedov vs Tony Ferguson

"How long must we wait?" is the question asked by every UFC fan about Khabib Nurmagomedov vs Tony Ferguson. It's undoubtedly the single most anticipated match-up that UFC can make and as you know, it's one that has been cancelled on 4 different occasions.
This is the fight to make and there's no contender who deserves it more than Ferguson. All injuries discounted, we may finally be headed that direction after over a year and a half.