In many ways something like
Bonus Round is just better than Krark. Bonus round does cost 1 more and doesn't persist through multiple turns, but since the doubling from Krark you'll want spare mana if you try to go off using him, and doubling through Krark effectively costs mana. I suppose that Krark could also be added to lists after Bonus Round #4, but that seems a bit of a reach.
So, if we really want to try to make Krark make sense, we've got to look for stuff that Krark gives but that bonus round does not. That's going to be cast triggers (
Jeskai Ascendance,
Runwaway Steam-kin,
Arclight Phoenix and the like) or some kind of approach that can exploit the value from Krark over multiple turns without being troubled by the reliability or mana cost issues that he causes.