Thought of some other things! Though they may not be the more popular series, since I really don't know what's popular right now. >>
Anywho, Descendants of Darkness/Yami no Matsuei is one that I haven't seen much merch for. Shaman King too, but I don't know how many people are into that one now. >> My friend Hashi also suggested Kyou Kara Maou and Kaleido Star, which I second. ^^ And there's always Gundam Wing merch that isn't the actual Gundams, but that may just be me. XD
Again, I don't know how many other people are interested in these series. Just throwing some ideas out there. ^^