Yeah I'm not really taking his age into it.
I'm not 100% on Barnett, but is he a wrestler? Either way, people like Mir are suited for Cormier. Mir is going to get battered on the stand up and will be looking for the sub on the ground. He can pull off anything, as seen against Nog, but I think he won't be able to lock it in as he's been quite lucky with submissions at times and Cormier won't make the mistake.
I think someone like Big Country is a good fight because it'll be a main stage fight. Maybe a co-main event on PPV. Roy can knock anyone out, so Cormier will have to make sure he thinks about everything before he does it. He'll probably use his striking mainly because he will want an impressive debut, and every fighter stepping in with Nelson accepts the "Knock out the fat dude challenge", so he will be trying hard with his punches (which will be a massive help going in with Dos Santos down the line). Although he's really strong and great at striking, I think he has a long way to go if he wants to box with Dos Santos or keep Velasquez away from beating him by punch ****.
Barnett is 100 times the fighter Nelson is. his grappling is better and he knows how to strike rather then just throwing an overhand right all the time.