I think you could add at the beginig the option to let the game randomize, and an option in case you want to make your own character.
There could be different beginings, like an enemy warrior captured for the arena, a slave who made angry the wrong people, the roman rebel who goes against the empire, the person without future, who hopes to win fame and gold...
They could merge into a more or less simmilar story body with subtle changes so the workload isnt too big, and maybe your choices decide if you go Champion, try to kick the ones in power, escape the arena, or end as kitty food for the lions.
You could even add a second/third force near the end of the story if you want, where the gladiator can choose (or just go along) depending of their choices, against invaders while they ally with Rome, against Rome allied with the invaders, or against the invaders that allied themselves with the old power in Rome that the player kicked.
Even if it is just a "select a guy to stab" I without any story at all, I would play it for sure. Its a weak point I have xD