Oh man, haven't thought that far ahead yet!
Crap, I'm going to have my work cut out for me avoiding DB Super spoilers in the dealer's hall, won't I?
Hrm. More seriously, probably looking more towards higher-end merch. Animation cels, limited edition statues (I swear, First4Figures has permanently skewed my sense of figure pricing!) possibly various manga, especially the types of places that offer bulk discounts. For some weird reason I almost never order manga as they come out off Amazon or whatnot, but at-con I'm more likely to binge-buy an entire series if I can. Can never go wrong with t-shirts and apparel, either. I almost always come home from Fanime with 2-3 new t-shirts!
As for series, just, a good spread. That was one of the things that contributed to my ditching Anime Expo in favor of the drive north; the AX dealer's hall was all just same handful of things for the same handful of series. (At the time, Bleach, Naruto, and One Piece and good bloody luck finding anything for other series!) Especially if there's a good mix of the new and popular AND the older and more niche.