I have thought about this too, and wondered if this would work...
1. Posters are actively encouraged to choose am appropriate thread icon.
2. Ideally it will be mandatory when submitting a new thread.
3. Mods 'adjust' the icon if it is mis-applied.
4. Provide some tool for users to view only threads of a certain icon, thereby enabling those who want 'football only' to get that view, whilst not changing things for those who like the current smogasbord of threads.
I'm not aware of either 2 or 4 being available as modifications, but I could pay someone to create them - shouldn't cost too much.
Belatedly, I've sort of implemented the above. At the top of the Big Board there is a new filter which will help you select any particular flavour of thread you only wish to see.
The results are limited at the moment, obviously, as the thread categories are new.
I welcome all feedback and suggested categories/icons etc.