Defias Brotherhood RPPVP is balanced, but EU RPPVP servers have low pop zones means there is not much world pvp while leveling alt.
Outland had active WPVP guilds on both Horde and Alliance in WOD and MOP, now not sure, but still best bet is probably leveling Horde Rogue/Druid on Alliance populated server like Outland not for guild wpvp but for 2-5man wpvp in group with friends.