Allow us to donate or sell thralls (1 silver to 1 gold each) to a collesium north of jungle (future DLC upon my suggestion). Let us enter the said collesium and watch our former thralls (randomly chosen) batter their way to glory and become a legend on weekly hall of fames board. Hell let us watch and/or bet on them.
Disclaimer: I suggested this very thing on another thread as a reply. But for better exposure created a new thread.
Not sure how well the hall of fame part would work.
I’d rather see a player-created coliseum we can stick at our bases. To battle our own useless thralls against one another. Like in Shadow of War.
And instead of one coliseum for PVP thrall fights, several in neutral cities around the map. So it’s not a pain in the butt getting your thrall TO the fight in the first place.
So you can then pit your thralls against other players. Strip off all their gear, they get a random weapon assigned by the coliseum scripting, same damage level but can be different types.
I’d like to see player built coliseums too, could do three tiers T1=1v1s T2=team fights T3=beast fights.
I like your idea about player-created colosseum too but I suggested a place because it gives an incentive for players to meet face to face. Of course PvP should be turned off at the colosseum, unless you ARE the participant of course 
If you care about lore, make it a sacred place where even the most barbaric of barbarians agree not to shed blood on its ground unless agreed upon in advance. As for the hall of fame part there could be a scroll sitting on top of a plinth or engraved on wall.