If I were going to use concentration as a limitation on a martial ability, I would probably want it to be an ability that drawing aggression is part of the deal.

Things like difficult terrain around the character or limiting anothers actions.

Concentration's only inherently useful trait is that it encourages enemies to target the character to break concentration. Spirit guardians is a great example of a spell that works like this, the damage and movement reduction are oppressive tactically which means a cleric can force enemies to attack them instead of other PCs.

Something like an action and concentration to 'prep' a free action surge could be neat as a holding action. The fighter will rip apart any that close if they aren't dealt with, holding enemies at bay with shear presence is useful in combat and on brand for a tank, and attacking the fighter in melee is a favorable scenario in comparison to a squishy.

But this is to keep in mind, if the goal is buffs, give buffs. No need to make it more complicated than that.