To be honest I don't see World of Tanks as MMO. The numbers playing on one map don't allow for the "massive" aspect.
I have to agree with Bliss in so far that the arena concept of Warbirds or Aces High (dating back to the infancy days of the commercial internet and Air Warrior) is no longer the role model to follow. For that the gamplay these arenas offer is simply far too shallow and one-sided: all you get is fighters gang-banging with a handful of bombers thrown in to provide a temporary variation of the everlasting
Take-Off - Furball - Die - Repeat. I've played Warbirds for a while, many years ago, and the arena gameplay became utterly boring very fast. Anyone played the old WB version 2.XX? Remember the MED map and the two fields, F4 and F9, and what happened in the middle between those? *yaaaaaaaawn!*
The scenarios and events, however, were something else and provided the most memorable moments of my flight sim career. Watching a long procession of B-17s on their way to Berlin while lining up with our flight for a frontal attack, knife-fighting Kittyhawks in my Bf 109 F-4 over Africa ... The old bulls in the MMO simulation market do give some of their users admin rights in some arenas to make such events/scenarios possible and with the utter flexibility CloD with its use of scripting can provide this could lead to much more immersive battles than ever before. But:
Such scenarios were run once a few weeks or even months since the time to set them up was considerable. For everyday ops they weren't useful (as too many players weren't interested in the discipline they required). I, personally, would strongly favor an arena concept that stressed teamwork and global goals for each side by creating and maintaining a fluid 24/7 world and dynamically generated "missions" based on current frontline needs or a player-generated mission with a specific target so that players can fill "slots" instead of going off alone to seek the closest gangbang. My aim in flight sims is to get as close as possible to the kind of missions flown by the real pilots, a bit like reenacting if you will, and this may be a way to create such an environment. I dunno ...
But right there my scepticism comes into play. To develop such things one would need to spend considerable time and "braincells" on gameplay aspects and so far no Maddox Games release has shown promise in that regard (with CloD being the lowest of the low campaign- and single mission-wise). So far MG has proved their own reputations: engineer-developers with a tendency towards the technical side of flight sims but no idea about decent gameplay.