Prospective Vayne/Fiora changes?

So i was thinking maybe they should change the way this type of damage works, at leas philosophically. Rather than it being a basic ability, what if they shifted that power to their ultimate abilities. For example, Vayne's w passive and Fioras passive would do physical damage rather than true damage, keeping the % Max health. However on using their ultimate abilities, those sources of % Max health Physical damage would switch to True damage in Vayne's case for the duration of her ult, and Fiora's maybe a bit longer. As well maybe in Fiora's case, as she pops vitals during her ultimate, she could gain more time with true damage, in the same way her ultimate heal gains additional time as she pops vitals. this would allow them to deal damage and finish tanks off, but would also allow tanks the chance to itemize a bit to last a bit longer. I'm interested in feedback on this idea, did i miss something, will it break these champions (either make them bad or too good), or is there a better idea.