I've seen a few gladiator games done, in a few different ways, mostly involving the players controlling a team of gladiators, being a gladiator, etc...
Made me wonder. What about using monsters as gladiators? (Perhaps gladiator isn't the right word, but the idea is there - monsters fighting monsters).
Everyone would make a character like normal, and this character would be the owner of his or her own house and small monster pit. Instead of buying a bunch of first-level human warriors and fighters, you buy monsters and pit them against each other. You buy a batch of recently captured goblins, maybe a displacer beast, and sic them on your opponent's beasties. (Reason: Maybe slavery is outlawed, but the law is poorly written and allows for slavery of monsters instead of humans, elves, dwarves, etc... ).
The player would control the monsters in combat so long as he adheres to their mental ability scores. Animals would kill what they need to (possibly the closest creature depending on how hungry they are) while mindless creatures follow commands given before the battle by their masters.
The point of the character having a sheet? For everything out of the battle. Perhaps a Profession (Promoter?) check to drum up interest in his team for a fight. Perhaps he uses Stealth to sneak into another person's place and poison their creatures before a big bout.
You could even, as you gain levels, go out and capture your own beasts to acquire things a little more exotic. In the meantime, a cohort you select can control things in your stead (essentially allowing you to still do the battles, the behind-the-games things, while hunting (as a group, or solo, or with a bunch of people you hired... maybe even some monsters you put through the arena?).
The idea of using humanoid gladiators is interesting but I think it might get old... expanding it out into the entire monster manual though? Capture a young dragon and use it to fight your competitors, rising through the ranks until maybe an iron golem takes it down. Perhaps you get a group of kick-a** kobolds that rise through the ranks and take on things not meant for them.
Roleplay Opportunities:
A "Viewing Box" thread that overlooks a given pit/fight, allowing people to observe the fight (in-character), comment, boast, and the like.
Any behind-the-battles intruige, perhaps a secret meeting with a competitor to join/rally against another who is dominating the arenas?
Adventuring when trying to locate new creatures or beasts, perhaps you run into people along the way who you can try to recruit?
Rollplay Opportunities:
Use profession/craft/perform skills to make money.
Sneaking in (either yourself or people you've hired) to another person's barracks.
Adventuring always has rolling.
And of course - the arena itself, where your monsters duke it out against others.
Gaining XP:
Win or lose; both you and your beasts gain XP for arena fights.
Roleplaying.
Adventuring, in-city or out.
Acquiring Wealth:
From fights your beasts won.
Perhaps you make a bet against (or for) your opponent's creatures, winning indirectly?
Adventuring always has it's own rewards.
Selling creatures, perhaps one has grown too old to be useful.
Pure profit from arranging fights between your own beasts on your land?
I mean, it sounds like there is a lot of possibility for everything yet centered around a single idea: monsters fighting monsters.
Everyone would start out at level 1 with a small plot of land (probably a 1-story house, a pit with some cages) and as they gain wealth and fame they pay to grow their land, facilities, and array of creatures. Everyone would probably start out with some kind of beastie; a group of goblins, orcs, kobolds, low level stuff.
Monsters, I think, would be allowed to level. They'd only gain XP from fights (unless you bring them with you) and gain more racial hit die or class levels (depending on the creature; no Octopus Rogue(s)!).
Fights wouldn't have to be strictly to-the-death, of course there would be more reward in those. You can outfit yourself (and beasts) however you desire.
Thoughts, opinions, things that should be considered?
Disclaimer: I probably wouldn't run this, and if I did, it'd probably not be without help. Former of me not doing it would be more likely though. Feel free to swipe. ;p