OK, but FOR ME, I don't see how 5e maps well to Mythos cosmic horror for instance. PCs are powerful competent adventuring 'heroes' in 5e. I mean, aside from being level 1-3 they are pretty potent, always have a lot of resources to call on, magic, etc. They aren't really easily going to feel threatened. Obviously you could just make all the Mythos horrors level 15+, but that just makes the problem worse, because the PCs won't really face them until THEY are up to high levels, at which point they are even less like a typical Mythos protagonist. Its also a bit hard to see how most/many PC class characters are going to be surprised and horrified by cosmic secrets when they are already wizards, warlocks, sorcerers, eldritch knights, etc. Mythos magic is very different in feel from D&D magic too.
I mean, I think you can do something that can be qualified as 'Cosmic Horror in D&D genre', but it will lack signature elements of classic CH genre material. OTOH it isn't like HPL is the last word in the genre. Even stuff that references the Mythos, like Laundry Files, does spin it a bit different in some respects (the Laundry Files protagonists are certainly a lot more competent and successful than any HPL character, though August Derleth did presage this sort of scenario a bit).
I guess I'm saying I have mixed feelings about what to even call it, and there are MANY systems I would base a Mythos game on long before I would pick 5e.