Server transfers from PvE to PvP should be unrestricted, period. The only real con I see is people leveling toons on a PvE server then moving them to a PvP server ( to avoid being ganked, no doubt. ) But, as someone who has played for 15 years consistently, I don’t see any real harm in this.
Go ahead Blizzard and make it a thing as of WOTLK Classic or sooner ( the better )
~ Fel