Seen this idea since Shadowbringers and I'm not against it per se (I've even suggested it myself often enough) but it does reduce the engagement possible from TBN even while better securing it against the problems of excess ilvl.
Hmmm...
Another I've considered is to have the barrier be variable with MP available (spending 30% of current MP), greatly amp bankable MP generating actions, and maybe both have DMP (from damage absorbed, usable on MP-spending attacks or temporarily adding to attack power) and, say, VMP (from MP spending attacks, usable on drain actions or to generate Attack Speed), so that you can slim or beef up your TBNs. Still working on a mock-up for that though.
