Casuals dont play around with specs because the game is too restrictive around swapping classes and specs. Every time you want to try a new spec, bam, gold cost. Every time you want to try a new class, bam, level up for hours upon hours.
Not to mention both require re-gearing.
If you want more arena participation, allow people to make pvp only max level template characters who have free access to all the gear and enchants and gems in the game.
Then make those realms free to play so anyone can play on them.
Then create a reward structure around unlocking alternate gear visuals for your characters instead of power progression.
Congratulations, now you have a proper pvp system.
As a bonus, make pvp specific class balances that only affect those pvp realms, so you dont need to try and balance both pvp and pve at the same time.