That's a really tough one. I would think that dividing up by genre would be best, since dividing by gametypes would be really hard. (Is Shadowrun a more like WoD or DnD?)
Horror, Fantasy, Sci-Fi, Mundane, "Strange or Other"?
However you end up dividing, I would subdivide by One Shot/Short Form vs. Ongoing Campaign if that's an option.
As for icons... I could put some stuff together. Maybe a Cthulhu silhouette for "horror", a Sword for fantasy, a CyberEye for Sci-Fi, and a pistol or a normal face for Mundane?