The trouble is there is already a function to do this - its just that people dont use it!
The chances are if there were separate forums, people still wouldnt use them correctly, and the mods would have to spend even more time moving things to the correct place.
What should happen is when you start a new thread there is an option to select a prefix: "Original; Extra Easy; Red Days; Green Days; Extra Easy Days" Actually - that could do with updating as there is some repetition there with the Original/Red and Extra Easy/Extra Easy days. And I'm sure there used to be a "Puddings" one?
I'm not sure this idea is a go-er to be honest, there are a ton of sub-forums already that arent used properly, but it would be good to see better use of the prefixes.