Using technology from multiplayer on-line games, I think you should be able to connect to other people's projects if both people desire to do so.
Each person has a separate clock, so syncing playing doesn't have to be fretted over, however it could be an option. Users can work on different sections of a song without distracting eachother, but still be able to see and listen to what the other people are using. All information is updated realtime, and changes can be seen by other users as they are made.
The program should allow a couple features:
2 timelines per user - one is a scratch board, which isn't shared online. The other is the master timeline where people can copy over patterns from their scratch board. Users can also directly work on the master timeline.
Online syncing and uploading of 3rd party VSTs - If one person uses a VST that the other doesn't have, it will upload it to the other person. this could be a problem with paid plugins, in which case the person who doesn't have the plugin instead gets an audio track.
Save history - somebody crash the boat? load up an older file and keep it going.
Color coded work by users - See who has contributed where. Maybe even a live feed log window that can be viewed.. i.e. "User47 has just edited pattern 16" "User47 has renamed pattern 16 to DrumFill3". this can be mixed with a text chat as well.............
Online voice/video chat, as well as text.
Better notation on timeline for communication - If people are working at different times, they may log in to see clickable icons on different sections of the timeline. Clicking these will allow users to view notes left by other people.
Screen sharing - This may sound redundant, but if you want to teach somebody how to use FL-Studio, taking over their mouse and keyboard inputs might be a good way to show them examples.
Section Lock - and optional switch which can lock a section you are working on so it can't be edited by others. No drama, please. Plus, it'll help people be more comfortable while working with strangers online.
On-line community - A website with a list of users and their talents. Find somebody to collaborate with, and publish your tracks online to be viewed by anybody.
Hosting locally and online - A local host might be more convenient, but a premium feature could allow for hosting on an official image-line server.
The main thing is having multiple people work simultaneously.
LAN mode - like rewire in a way, but optimized for people on multiple computers in the same room. Synced timeline, Master output (1 set of speakers, not 3 or 4 blaring over each other). Prevents hot seating which can be frustrating and ruin a vibe.
That's basically it. It's holiday season so I'm just writing out my wishlist.