The simple solution is to scrap the passive trait bonus. Passive effects are not exactly awe inspiring to start with. It frees up IB use and flexibility (which is far more important than a passive to play around), is one less factor in the damage buff from spending dual gauge (though simply adding adequate damage to the reward either in +crit rate or in any form whatsoever will win out. The only requirement is that the number for spending the gauge outweighs the cost of obtaining the gauge and it automagically works.)

It doesnt have to be this specific idea. It can be anything. The only requirement is that IB rewards you by indirectly increasing damage more than the cost of using FC indefinitely without simply reversing the problem to only use IB and never FC. Any type of system that requires you to use both to obtain a dps boost over using 1 alone will work. The simple solutions i came up with is just to require an equal number of each and then unlock an action that makes up the difference and then some. It sets forth a long term build up and release system that invokes the old Berzerk/IR windows of 4.0 that I really enjoyed. But any system that creates synergy from using both actions will get the job done. (assuming we are unlocking IB from tank stance and working around that new norm).