I think this would be a great idea, but for it to work properly there should be a few rules:
1. People can choose to enter the united server and bringing their whole account from their server. This is because active players may not want to join a new server and start over unless they can bring their existing account with coins.
2. Haypi actively prune this 1 server, no login for over 2 weeks = boot and their account can just be transferred back to their old server if you will. This will be complicated but it is only 1 server.
3. People may only get onto the united server with certain level of prestige and achievement points similar to pvp, however the server is not going to just reset after 15 days.
4. The current servers all exist but the main purpose is to allow new players to learn the game before they enter United Server.
5. There should be some more ways to limit alt accounts on the server and a punishment system for those whom are caught with alts (eg. if two accounts are been logged in by the same device for a months and no other, then chances are it's an alt and not someone babysitting an account.)
6. To prevent people just taking all their alt accounts to fill the server at the start, I think the requirement might be a minimum login time of say 5 hours over the past week before you can join up, and not just 5 hours of haypi sitting and playing in the background, to quality there should at least be some sort of command input every minute. I don't believe this is too hard to meet for the active player's main account (I do 5 hours a day easily

)
I know this will be very difficult to implement, but I would like to see a server that will continue to be thriving and active,as well as to have to opportunity to play with all the players across all the servers.
Also I don't believe this will have too much impact on pvp participation rate, pvp is only run as a 15 day competition and you must start over which will be completely different to a united server.
The only problem I see will be that all normal servers will be alot less active and this may not be good for new players, but then I think just to combine most of the servers and limit them to say 5 or 6 for new players to enter and develop.
Then if the united server ever fills up (which I doubt will be anytime soon), haypi can be sure it's at least filled with mostly active players before opening a similar server.