I just started a "team effort" in earnest. Met a guy online years ago on the VSPlanet (much like here; musical, social...) He invited me to play sax on several of his songs which I contribute freely and happily.

Recently he suggested we collaborate in the most pure sense....literally both work. I tossed in some changes I'd assembled with no melody, he asked for the Title and the Theme and wrote some lyrics and a melody. I'm producing, we both arrange.

One mostly done, the second in the nebulous stage.....quite fun.

But Eddie....to your point about "having a sound in your head"...I won't contribute my stuff that I've got plan for....I just contribute the sketchy stuff...motive, string-of-changes that haven't coalesced. I'd recommend you break out your stuff into 2 groups like that and contribute one "rough idea" to a collab w/ a pal. See where it goes. Its quite fun, and I've felt the sum of the 2 so far have exceeded the value of their parts.

When I worry that I'm letting something go that I might miss, I recall the quote of some famous composer (name escapes me) who, when asked how he came up with so many melodies, answered "the trick isn't to think of them, the trick is to not step on one when getting out of bed".
They're everywhere.......

One important part is the relationship; trust, respect, ability to be honest without cocking up the whole thing.

Best of luck.