I love Tahm Kench's character, and admittedly his design is supposed to be both comical and intimidating, but so far his only skins are him as a chef and him dressed as Urf, which is no problem in itself. What I just hope for is that his next one tips more on the fact that he's a dangerous demon and less on him having a funny appearance. There have been a bunch of ideas that lean towards this from artists, which I'll leave below.
Not exactly "serious" per say, but it still retains a creepier appearance.
This one just works, given the nature of black holes as eternally hungry for everything, and Tahm's own need to feed.
Similar to Dark Star, but with a slightly more animalistic and alien feel.
One more, that's a bit more my own personal idea, is Candy King Tahm Kench. While it doesn't sound that serious based on the name or immediate appearance, like Ivern's, it's based on Tahm himself looking to eat everything, fitting into his overall theme.