Unlike Kaido, which has a wingless Dragon DF, Nidhogg is a dragon/serpent with wings, and with a much darker color, so I expect a typical Dragon design, which I like very much, or much more than wingless goofy dragons atleast.
Kinda curious if Oda will follow the lore and give him special decay powers which would be similar to Juku Juku No Mi, but I expect more powerful, and maybe even that's why Oda didn't used very well the Juku Juku.