It is very easy to maintain max stacks of Kalla’s fervor even without flanking. This makes it a very boring mechanic. It also makes Heroic Command (the F2 skill) completely useless. Another problem for revenant is the energy management, especially with the huge cost on the new skills.
So here’s my suggestion;
change F2 to a new skill with no cast time, no energy cost and short (maybe 5s) cooldown that reads: Consume your Kalla’s fervor and gain 10 energy per stack.
That way building fervor is actually interesting and it will give revenant some new way to gain energy, which might mean we can swap legends when we want access to their utilities rather than whenever it’s off cooldown to regain energy.