Take the Crystal Scar, remove all the walls and terrain so you have a giant circular Colosseum. Maybe add some circular columns around for things like Talon to jump over, or outplay for other champs.
Maybe limit the champion pool (like the blood moon event), so no hard tanks, hard supports, and hard mages. No tank items. Prefer to keep some adc's and more fighter tanks to have some diversity, and so we can allow champions with themed skins as an example (Gladiator + Beast hunter Draven, Beasthunter sejuani, Barbarian Sion). More bruiser and fighter focused, rather than assassin focused.
Have jungle monsters spawn for players to kill for points, or have bot's spawn as 'monsters' (Prehistoric chogath and renekton, some Mega Gnar's, sandscourge Skarner maybe)
Just my ideas on how I could imagine such a Gladiator arena type mode.