I have read multiple time that EHG might consider to keep the arena mode even if it was not planned at the beginning. You said that you don’t want to have an arena mode that is always available where people could stay all the time. So here is a suggestion how to handle that !
Arena could be accessible in the End of Time by using a “Coliseum Memory Shard”. These shards could be found as rare drops for example. Those shards will have some random affixes that would change the difficulty and rewards of the arena (a bit like PoE map’s affixes). The arena would not be endless, a maximum number of waves with a boss at the end. When you manage to beat all waves, you get a reward at the end. Those shards could also be tradable in the bazaar.
Lore wise, those “Coloseum Memory Shards” could come from ancient gladiator fights that happened in this coloseum. You travel into the past to revive those fights and show to the crowd how powerful a time traveler is. Depending of the affixes the Shard has, you will end in different eras, different difficulty, differents monsters and rewards.
And for people that just love arena fights to try builds, maybe let the possibility to start an endless arena mode but without any loot, just a leaderboard for the highest wave count achieved.
What do you think about that ?