Make them uninterruptable to hard CC while casting their ults and only during their ult cast time.
So Darius cant be stunned while dunking in mid air.
Illaoi cant be stunned and interrupted while slamming her idol down.
Garen cant be stopped while summoning his sword to dunk on fools.
LegenDariusGaren2 votesGaren already can't be stopped if he presses R. You can press R on 
even if he is out of range, and when WW blinks to you to suppress you, he will get dunked while you are cc'd
Big Lincoln1 voteswe don't need to buff any of these three champions lol
AsphodelNext1 votesFrom the wiki: Noxian Guillotine(Darius' R) has a cast time and cannot be interrupted by anything other than Darius's own death. He will still suffer any crowd control effects, if duration permits, after the cast time ends.
Pyrithegethon1 votesOh thanks for showing that, i thought i was being interrupted when ulting as darius.
Is that the same for Illaoi too?
AsphodelNext1 votesCouldn't find a quote for it, but it says it's point blank. Most abilities, unless they use the word "channel" instead of "cast", can't be interrupted except by death (Lux ult, Ezreal ult, Jinx rocket, etc.)