An app where amateur musicians can post musical ideas they want to realize and invite people with skills and instruments that they lack to participate. The collaborators don't get paid, the motivating factors are interest, fun, and exposure (through giving credit to all the participants in a publicized recording/video).
Find people with musical skills and instruments to help you realize your musical ideas that you can't execute on your own.
Motivation for many amateur musicians to engage in music. Many people who used to play an instrument, sing, write lyrics, or whatever musical activity they did, tend to abandon it with time if they don't have external motivating factors. Seeing musical projects that excite you and that you'd like to participate in can revive that motivation.
Meet like-minded people and make friends.
Gain exposure - the initiator posts the recording on youtube, soundcloud, or elsewhere and gives credit to all the participants - that's the reward besides having fun and meeting people.
There are many apps for finding musical collaborators, but they mostly work as hiring platforms for freelance-type gigs or/and the collaboration is happening online. My idea is to have an app mostly for amateur musicians and motivate people to collaborate live and without money. Here's a good explanation why online collaboration is not a good idea apart from live one being more fun.
Whenever one gets inspiration for a particular musical project, be it to perform and record a song they've written or make a cover/remake of an existing song, a specific jam session, or anything else and they need instruments and people to play them, singers, etc., they post the description of the project on the app, attach a sound sample, list what they lack and what each person should learn to play. The app is geolocation-based so it shows you musicians/projects in or close to your area.
I'll give a concrete example to make it easier to grasp the process: I want to make an acoustic cover of this EDM song, a sort of orchestral version of it, cause its sound fits for that. For this, I need acoustic instruments to replace all the electronic sounds - percussion, brass instruments, wind instruments of the violin family, and possibly also vocals. I go to the app and create a project, describe what I want to do, what I need, and link the original song. Other users then review the project and some of them (hopefully) mark themselves as willing to participate.
It would be best if the app had an inbuilt means to dissect the song into separate components and sounds. That way the initiator could indicate what exact part they want you to perform. It might be complicated to pinpoint this using words. Then the people interested in the project could listen to the separated sound sample over and over and learn their part before meeting for the rehearsal/recording. Ideally, the app could also automatically generate sheet music for each separate part based on the extracted sound samples. The potential collaborator can then review the sample/sheet music and decide whether they are competent enough to perform it.
So the vision is to have all the required information in the initial project description posted by the initiator to allow those interested to learn their part on their own. Those interested can also clarify the details with the initiator through comments or direct messages.
The initiator later reviews all the proposals and selects whom they want to collaborate with the most. Each user has a profile with a short bio, skills, reviews, and samples of their musical work if they have those. The potential collaborators can prerecord the bits they are expected to play and send the recordings to the initiator so that he/she can evaluate the quality. Those who'd send the samples would have higher chances of being selected.
When all the collaborators are selected the group decides on the meeting time/place and works on the project. Preferably all the musicians would learn their parts on their own in advance so that the recording could be completed during a single meeting.