Hey Danny,
certainly a valid idea on a possible future feature :)
Let me just give my personal feeling towards this, maybe some others would like to share their opinion so we can decide wether that is really a good thing to have.
Personally, I have seen such a "request remix by instrument"-flag system on some other jamming community (otherwise quite un-like to the 'loops btw), and it created a somewhat strange impression:
By seeing a lot of tracks that are "requesting attention", one gets the impression that there is a lack of attention. I know that this is a common feeling among creatives, independent of the platform or kind of release (CDs dont get the deserved attention either...) which can not be avoided, but I felt the "request" thing shifts the focus towards the issue in a negative way.
New people might be lead to believe wikiloops worked by dumping material and adding a wishlist, which is far from what we do.
Lastly, how to treat the "request" flag once th request is met? If I'd like to have some vocalist on my tune, but the first vocalist who ads really ruins the song IMO, will the request flag disappear? Will it stay? Shall it be time based, p.e. for four weeks after release?
The way wikiloops is set up now only lets you do one thing:
You may share your work if you like, knowing you have no idea what will happen and no guarantee anything will happen. You are happy if someone gives feedback or has fun with what you made.
I believe its really good that expectation-free way, and it makes it really easy to create the positive experience of "more than I would have expected" :)
The request feature might lead to the opposite. If you request bass, keys, sax and vocals and get a really outstanding bass remix, you may still feel only 25% of what you wished for happened.
If you couldn't request anything and get the same outstanding bass remix, you might appreciate that unexpected thing much more...
I know people don't really like to face it, but there is a lot of psychology to maintaining a community in a good spirit. Unlike some other sites (...) I try to steer away from manipulating emotions by artificial semi-intelligence, but avoiding possible psychologic traps is something I don't feel bad about.