While I'm not trying to dismiss/disregard your idea, I think its way too early to even think much about this, and I will give you an example from RO.
Champions could, in theory, oneshot almost every other class in the game with a specific skill, said skill required preparation, gear, a specific build and lots of consumables in order to work properly and not be completely useless.
Some jobs like Lord Knight/Paladin had passive skills that had a chance of completely negating an instance of damage. Professors could dispel the champion and brick the setup required to use said one-shot skill, Mage Archetype/jobs and Priest could plop down an area barrier that would negate the next few melee hits, which would make said skill miss. Dancer and Bard had an ensemble song that would negate skill usage in an area. Alchemist and Blacksmiths could break weapons/armor, Stalker could force unequip weapon armor helmet and shield.
All of these would be passive to counter or highly diminish the Champion's capability of one-shotting people, if said skill were reduced or toned down for "balancing purposes" or "fairness" said skill would be dead in the water.
New items and jobs added over-time will naturally cause shifts in the meta, odds are you'll have jobs that excel at countering another job, only to have yet another job be an excellent counter against the 1st mentioned.
So i'd wait a bit till we have a few more jobs before suggesting this