Ian, like Jon said, having so many characters on screen with individual AI isn't worth it at all. If there are that many enemies, the game would just be a boring hack 'n slash, but even worse would be the camera. You would have to zoom out so much that it would be hard to tell what's going on, and I don't think it would be fun at all. That's what you would give up to make it seem more epic. A 30-man Royal Rumble would be nice, though, but then it leaves a lot to luck. At the beginning it would be extremely chaotic and chances are most people would just button mash. Therefore, the more skilled players could be tossed out early without anyone actually beating them. As more players get thrown out, more skill is needed as it is less hectic, but by that time, many of the actual good players might be out already.
As for normal matches, you have to take into account the length of a match - if there were, say, 8 people in a normal match, it would never end, since only one player is needed to break up the pin, and the ref has to come over first to count it, which would give even more time for it to be broken up (if you're not using an auto-ref). Unless you want to do it like the Smackdown series, where no hold or pin or move can be interfered with, which I absolutely HATE. How unrealistic is that? You're standing there but you can't break up someone's special or pin. And you can't even release your own hold when you're doing something. If someone is climbing the ladder to get the belt, but you are putting someone in a submission, you can't let go of the hold to go after that person! What a joke..
In the Day of Reckoning series, you can let go and you can break up anything at all. This is infinitely better, and I would think that the game you're thinking of would have this option. If it does, then with 8 players a game it would take a long, long time to end (if it did at all).