Cain is a small HW.
Machida could do it. But he's a MW now.
Phil Davis would be a good fight.
And no - they wouldn't need to gain weight. Let them fight at that 225-230lbs they're already weighing. It wasn't problem for Cain, who's 240lbs or even more so for Fedor, to fight guys even 50lbs heavier than them, so let's not make it an issue.