Just throwing out some ideas my mates and I had in TS.
I know not all of them are feasible in WvW with current game mechanics.
1. Possible an expanded underwater objective or keep.
A domed area that requires maybe a sub siege weapon to break in.
2. Maybe throw in a mobile airship. Pretty much a device that can fire lets say realistically 2 cannons one on each side. It can be shot down from keep fire or auto attacks. You dont make it high enough to go over walls but enough for ranged attacks to hit.
Think of it as a heavy low flying ship that costs lets say 5 gold. Or you need to capture and objective to get it.
3. Roaming groups of legendary defenders at spawn stopping the spawn camping.