Woohoo that has me excited! Hmm I suppose it depends on how you design him. I know a lot of people when doing Merman sex scenes design them so that they have a front hole and a penis, so that they can be topped but also so that they can top as well, which is something I always find interesting, and I think that adds some sex scene diversity. Also, I'm not sure if you plan on doing any kink/ fetish stuff, but years ago I came across a CG comic where a merman becomes infatuated with a human male's feet because it's something he himself doesn't possess and he loves the idea/ concept of feet so there was some foot fetish/ foot worship scenes.