When a server has a queue for all four WvW maps, give them the option of participating in WvW on a different server. This other server should be chosen by the game and not the player, preferably not the lead server in its bracket, and never be one of the two servers that you are competing against (as in, never a server that is in your bracket). This suggestion draws very heavily from your implementation of overflow servers for PVE zones. Players who join such a WvW map should be queued into one of their own server’s battlegrounds while playing on this “semi-overflow” WvW. Unlike the PVE overflow servers, when your queue pops, you should have only two choices: join in your world’s WvW, or be kicked to Lion’s Arch.
It might be necessary to add further debuffs or limitations to such players to dissuade them from purposely and perpetually playing on these semi-servers. Perhaps they wouldn’t be able to talk in chat. Perhaps they wouldn’t benefit from orb bonuses. Perhaps they couldn’t build siege or perhaps they wouldn’t be able to loot badges. Likely, you’d have to require that they join only borderlands games, not eternal battleground games.
I put this suggestion forward with great hesitation, as it breaks one of the big design goals of WvW – the battle between different, distinct worlds. The reason I even bother putting it forward at all is because I think that it doesn’t completely destroy this design goal. The free server transfers currently undermine this design goal more, and when those transfers start to cost money there will still be server transfers for no other reason than WvW status. Limitations should be in place to encourage playing on your own server to keep this design goal prominent but not absolute.
I put this suggestion forward with the premise that it supports a higher design goal – fun WvW. It would let people participate in WvW even if their server has huge queues. It could be employed to balance out team sizes, if Anet wishes. It could lead to more active borderlands. It would lead, overall, to more PVP. It might encourage players to consider other servers with shorter queues that they otherwise wouldn’t consider transferring to.
There are downsides. It doesn’t really solve the huge queues on some servers. It won’t appeal to all WvW players. It can’t easily accommodate a large, organized guild group. It would probably negatively influence the scoring system that is currently used to rank servers. Depending on how it is implemented, it would likely reduce mobility between the WvW brackets. Servers who suspect that the opposing teams are benefiting more from overflow players will complain, regardless of how it is implemented. I personally think that these downsides are counteracted by the potential benefits, but I can understand how many would disagree.