Short, Sweet, Simple. You go to an NPC in your factions main capital, you sign up to become a gladiator, starts a questline, first mission, win an arena battle, you get a number of options that will always be available to you. Solo, Group, PvE & PvP, always repeatable, always available when you want to do it, if you wanna do it at all.

1. Beastslayer, you queue up for a fight in your home cities arena pit where you're pitted against an elite monster or group of elite monsters for you to solo.
2. Team Beastslayer, you queue up with a maximum of 3 other allies to test yourselves against a more difficult group of elite monsters, like queuing up to kill a single raid boss.
3. Bloodsport, you queue up to fight against a single other player, this player is random and could be from any faction including your own and from any Cyrodil campaign.
4. Team Deathmatch, you and up to 3 other friends queue up to fight against an equal number of enemy players, this can range anywhere between 2v2 and 4v4's.

Upon completing this quest you are rewarded a small sum of gold, and if fought against enemy players, a small sum of alliance points to be spent in Cyrodil where you unlock the next part of the chain, win a arena battle in an enemy factions capital city. Same reward, next follow up quest, win a arena battle in the arena of the final alliances capital city, upon completion you are rewarded, yet again more gold and possibly alliance points, the next quest is to win a arena battle in Cyrodil's arena, completing this quest grant a blue weapon of your choices, 1h, 2h, bow, staff, unlocking the final quest and final arena/map, Boethia'a Proving Grounds, this Daedric Prince has taken notice of your skill in battle, win 5 arena battles of your choice in Boethia's Proving grounds, rewards the Gladiator skill line, with skills and passives focused on defense and consuming stamina to use(Available for use anywhere, I imagine damage mitigation abilities and a much needed aoe taunt)

Every arena would have a unique layout and unique beastslayer enemies (Ebonheart pact arenas could pit you against Kwama Warriors, or Frost Trolls or Bull Netches, etc, not to mention nord, argonian or dunmer warriors) You level up your Gladiator skill tree by winning arena battles, after completing Boethia's proving grounds, any type of mob can spawn in any type of arena (You could fight a titan in the summerset arena) Enemy players, or you, could get a home field advantage for fighting in your factions arenas if your faction controls Cyrodil like say a movement speed increase, or hidden players are easier to detect. You could even unlock the Arena Grand Champion title for completing certain requirements (Kill one of each race in the enemy factions in your home arena, kill your own race, kill each enemy race in their own factions arena, slay a titan, win a certain number of every type of arena battle, 5 bloodsport, 10 TDM, etc)