In looking at the old forum postings on the VIP site, the suggestion/request for more category icons has been around since at least 2005. While I'm not a programmer, I've worked with other programs (not task managers, though) that had customizable icons since the mid-90s.
Forgetting the customizable issue (would be great to be able to add my own icons that relate to the categories I establish) since that's a more involved issue, what's the challenge to adding five or ten more icons to the current program? It would seem on the surface to be one of the easier feature fixes to do.