Arenas were advertised in main Onyxia thread, though it was not clearly specified that they would be available in vanilla.
There are several question-topics/comments on forum about it, but none got answer from the staff.
As far as i understand/guess - everything about arena from technical point of view is already done and ready.
Skirmishes were tested on beta and seemed completely fine and ready to me and i enjoyed it. I did make some bug reports about arenas in vanilla on beta and they got fixed, so it kind of suggests arenas were planned to be implemented in vanilla.
Arena ladder also seems to be already implemented (https://armory.warmane.com/ladder/2v2/4/60), idk how some players created those teams as i (and many other's) can't find arena team creation NPC anywhere (which obviously leads to nobody even trying to que).
If it's already implemented/available - please provide us information about arena team creation, if SoloQ is/will be available, if there are/will be any rewards (i personally think there no need for any besides ladder and maybe some cosmetics as nothing else sensible comes to my mind, except maybe some kind of +50 honor kills on win or something like that, but that's debatable), e.t.c.
If it's not - please implement it, it doesn't seem to require a lot of work to place arena team creation NPC in a sensible place where people will be able to find it (with no cost of 80 gold, especially if there are no rewards from arenas).
Additionally if it's possible - add SoloQ (doesn't seem to me that it would require much work either).
As far as i know - no other vanilla server exist(ed) with arenas with standard no-consumables rules and a competitive ladder. I think that it would be an awesome and unique experience to have.
Pvp on vanilla was always kind of lacking, as for many pvp players like me there is nothing to do once they finish gearing, and with current HK ranking system the situation in BGs is considerably worse here (i would even say catastrophic).
It should be good for the server financials as well, as i assume many players will want buy items with coins for arena. And i see absolutely no downsides from adding arena if no one is forced to play it (worst case - there will be bad queues).