Whatever is used for the "concentration slot", remember, Barbarians can't do it while Raging, Druids can't start doing it while in Wildshape, no gish can do it while holding a smite spell, and rangers can't do it while using Favored Foe.

My opinion is that if this feature is open to any class, it must be open to use for casters while not otherwise concentrating. If it is only open to martial classes, how does it work with multiclass into casters?

Some things to consider.