2.5k playercap is the best they can do.
If the server is full, players will go to the non limited PvP server (for pvp twinking or whatever) and the opening of the PvE server should not affect the PvP server population in a bad way
Regarding cross realm battleground features:
If the PvE server slowly catches up to the PvP server timeline, it would make sense for high levels.
However in low level brackets were realm pools are almost mandatory this would be a huge (HUGE) improvement