A bit late to the party as I just came back from vacation. I don't have any strong opinions on this, but let me release a couple of random brainfarts anyway.
First of all, regarding allowed filetypes, I don't mind either way. However, I think that if .pt3 is allowed, then .stc (SoundTracker) should be allowed, too. It's not as popular as pt3 nowadays, but still widely used especially by Russian composers.
I very much like the idea of splitting aym into aym classic and aym+. However, I disagree with SIDwave and samples being disallowed in aym classic. Samples are possible on standard AY/YM, not just by using the STe's DMA. Same for SIDwave, it's totally possible to do it on a stock zx128k. These tricks are just not being done because it would cost a lot of CPU time, which is no good for demoscene purposes (which is somewhat changing lately, for example sampled drums are used in several recent zx demos). Also, where do you draw the line? Is buzzer ok for aym classic? Both zx and atari can do it, but on the latter higher refresh rates are a common occurance. How would we enforce this? Is anybody here confident enough to determine whether a buzzer effect runs at 50Hz or 200Hz?
So overall I think the better idea is to seperate classic and + along hardware lines. Anything that uses only the bare-bones sound chip is aym classic, regardless of the software tricks being used. Anything that requires additional hardware (STe DMA, beeper, turbosound) is aym+. Aside from better enforcability, this probably also has the benefit of encouraging coding experiments.
Btw the .sap format has a somewhat related problem, as it will happily play back GTIA code (!= Pokey).