Quote Originally Posted by JLandan View Post
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.
The entire point of this thread is that casters get this whole resource, concentration, that fighters, barbs, monks, and rogues don't get the benefit of using. You responding to spitballed suggestions with "well it's only fair if casters also get this!!!" is entirely missing the point - they already get to do this! They have spells with insane ongoing effects such that the wizard or cleric or whoever dropping concentration might literally swing the battle against the players. Why would they even *want* more ways to use concentration.