Do you even understand why it's called a dunk? Ziggs is a definite no, and Illaoi is somewhat plausible but in the end, it's still not that good. Garen is legitimately the only other person who would legitimately be able pull off a dunk seeing as his ult is akin to Darius' in a way, which is how and why Dunkmaster Darius exists, his ult became known as a dunk throughout the community, and it works because it's is ULT, and becuase it's used to usually finish them off/embarrass them, much the same way a real-world dunk embarrasses whoever got dunked on. Vi could even have a Dunkmaster skin because of her ult.
Inb4 "that's not why it's called a dunk"
I understand that "dunk" was used to describe sure-fire things, or "insane" kills and/or outplays (Yi was one of the original dunkmasters because of this)
"If you are better than the person you are competing with you dunk on them." "To effectively beat someone at an event: used to symbolize dominance over ones opponent." If you've ever played and/or watched Undertale, you'll notice occasionally when you lose to Sans he says "geeeeeettttttttt dunked on" for this very reason. He's better than you, and he's emphasizing his dominance/superior skill.
Again though, Darius got the skin because one of his abilities resembles a dunk. Illaoi may be considered, Ziggs most likely won't, and Garen is likely being worked as we speak due to the high amount of requests/support form the community. Vi is likely to get this skin as well for the same reason(s) Darius got his.