So I know I'm VERY new here, but I think the thoughts of someone who has literally just joined may be useful for you all.
I think something really useful would be some key resources and indeed lessons to help understand instrumental idioms. I may be new to this site, but years of orchestration classes are what have made me into the composer I am today. Understanding real instruments, that are played by real people is essential to anyone who plans to write acoustic music. Just looking around it's obvious to me that there are people here with real talent, but that talent needs to be backed up with solid, academic grounding. I've seen pieces that are wonderful - in theory - if the flute could play forte in it's low register or if the violin could actually play a double stop on the D and E strings. So much of the talent here needs to be consolidated with good knowledge, a composer should know the principals of every instrument and having an easily accessible page teaching people about these instruments would be very useful.
My next idea is a little out there, but as a composer a live performance of my music is the best thing I could ever hope for. I wonder how many of you are also talented instrumentalist who could actually go away and record some compositions or even perform them! I'm very lucky to study in a university that is dedicated to performing student compositions, because a real person playing your music is so useful. So for you competitions why not give a prize, choose the instrumentation based on the performers on the site and then the winner gets a recording of their piece. For instance I could easily record a string quartet piece... it makes it harder with larger ensemble works I know, but in many ways composing for a small group of instruments or solo instruments is actually more difficult, so it could be a real challenge for the competitions!
Just a few thoughts from a newbie!
Adam