I would really love to see an "undertail" marking. It would pretty much look exactly like you think it does, it adds a marking to the underside of the tail, running from the rump all the way to the beginning of the tuft.

Colors could include the different colors "undermarkings" usually comes in, such as white, cream and silver and possibly even more! =3

Just a small marking I find myself missing once in a while. I'm thinking it should either be a common NCL marking (at least white and cream) or at least an Oasis marking.

Sorry I don't have a visual of this, I'm not an artist at all xD