Well out of the main ones
King X Diane - The best. King's entire life sucks, and the universe seems hellbent on denying him this one bit of happiness. They already like each other. This will be the most satisfying.
Ban X Elaine - He stays faithful to her even after she died. Still continues to look for a way to revive her.
Meliodas X Elizabeth -Generic.
Gowther needs someone. Maybe he'll get a goddess? Or maybe the guila thing will be revisited and he will try to properly apologize once he gains emotions and realizes how much of a dick he was to her, and that making her have sex with him was wrong, and then they slowly grow closer?
Escanor X Merlin - I don't know about this one. Nobody knows what Merlin is thinking and Escanor himself said he isn't expecting or trying to be anything more with her, and knows he likely isn't anything more than a interesting research subject for her, and so far I would say that is likely the case.
He might need to find someone else. Maybe he'll get a goddess instead by beating one, and not killing them and impressing them with his chivalry or something, and neutralizing them as threat with the power of love? And due to her constant advances he eventually starts liking her? Not being scared of him seems to be a huge thing when it comes to romance for him.
He did give Mera that option to live if she wanted.
We still have two men in the sins group with questionable romantic futures, and the next arc antagonists are all women.
So one of them has gotta be finding their true love here IMO.
Merlin might not end up with anyone since she seemingly has no interest in romance. So she just might go back to raising Arthur into a proper king/warrior, plus he has a crush on her too.
I'm not saying it'll go anywhere because he is still just a kid, but maybe when he's older?