I think you might be onto something in grouping games by types. As you pointed out, genre type would be good (horror vs romance), and maybe type of play (visual novel vs platform shooter vs exploration?), but also I think video game vs analogue game. I've known quite a few customers who specifically look for that difference in their bundles. The trick might ultimately be to see what groupings you get depending on how you divide things up.
Trying to think of this as a consumer even more than as a creator ^.^;; Hope it helps at least a bit.