Alvinophilia (navel fetishism)

It's not very a mainstream fetish but as Wikipedia said (lol), it's the second most popular fetish searched on internet.

It could be cool to add to characters an navel and to play with it during sex session, such as fingering, licking it, tickling it, rubbing the cock on it (so hey) and cumming. If the character has this fetish, the anticipation goes higher and gets more aroused and asks to continue if it feels good, or to stop if it hurts and don't like that.. It could be possible to use "toys" such as pen, needle, wax or something like that for increasing the experience.

The navel can be an outie or an innie and the experience could be different according to the navel's type.