More stats and more detail about fighter's styles and personalities.
The more the better, the more they'll seem like real people and the more goes into managing them.
If you meant you're going to make "clutch" and injury-proneness visible stats ...
. I believe they're in the current Version, but hidden, right? You get a pretty good idea about "clutch" over time -- it's that guy who, when he's down to 2 points and his opponent has 8, seems to win more often than not. Making it a visible stat when you sign the guy will detract from both realism and difficulty. (Real managers in any sport know, when they're considering signing a guy, whether he's fast or strong. They don't know whether he'll choke.)
And if having computer-managed "teams" to balance things out isn't feasible, how about just having a pool of subs (could be the same as the pool of the unsigned guys, especially if you make that pool a little larger) and have the computer plug them in for guys who are recuperating from injuries? I've seen good men die when they're fighting with a 45% handicap and their durability is very, very low, and that seems pretty cruel and senseless even for the gladiator business.