Each character has there own set path, P1 is always the path, having all characters co-op is the same as solo, makes no sense at all.
The fact that there are 2 players instead of one creates its own dynamic, and with the addition of new opponents during co-op Classic Mode, shouldn't it be given its own category?
Just my 2 cents
I disagree, as it is a much longer 1P run, it would not be correct to allow both players to pick say mario/dk, then go to link/samus completely skipping dk's path, adding in more characters each stage doesnt make enough of a difference to add it.
Of course Player 1 has to play every character's path, but Player 2 can select whoever they fancy, which gives co-op its unique gameplay style.
If need be, rules can be put in place that every character has to do their path and cannot skip it.
:)
It's a good idea to add co-op imo. Either make 2nd player pick the same character or let them pick whichever they want. Both are good ideas.