Gladiator have this advantage in arenas since he can simply reset his forward guards giving him an infinite loop of immunity. Lupa, Spiritwalker, and Invoker also receive additional SA when summoning Heilang or Tett without a cooldown. In a match where SA should be limited to three skills (normal arena), they break the rules.

In addition there are only few classes who have this extra auxiliary skill button received though their passives; Berserker's Unleash, Invoker Lupa & Spiritwalker's Summon, Gladiator's Shield Block.

So I'd like to suggest to give all shield classes like Nova, Paladin, and Lancer to have a skill like Gladiator's shield block effect. 

While those who does not should at least receive some auxiliary skills that can also increase a class's performance like Unleash. 

Like:

-A manual activation of when to activate all stacks of Kunoichi's Ninjutsu increase the maximum numbers of stack since she needs better damage. 

-A healing ward that consumes MP per second for Archmage. 

-Short distance teleport for Reaper that consumes a shard since she lacks mobility. 

-A new skill placement for Legatus's Lightning Sentinel that consumes MP per second while being summoned, and replacing the previous skill with his awakening grab. 

-Maybe Tiger Blade for Blademaster that ables him to use 2 swords that disables some skills but enhances basic attack damage while in Super Armor with long cooldown.  

-A new skill placement for Grandmaster's Energy Vaccum that consumes MP per second and has super armor, since Tempest Fang should always be on.

-Maybe a Ghost Step or transition stance for Sura that enhances next attack since he needs that micro movement and damage.

-A Tactical Strike that can be turned on or off for Fletcher that enhances basic attack range, damage, and applies knockdown which consumes MP per shot. 

-A Breath for Archer that throws a Spirit Bomb to heal or damage a group of targets which depends on the last skill used.