One big effort I made to improve The Mountains Swimming 4 horns and piano arrangement, to bring it up to snuff, was to create a section for the horn players to improvise, and another section for the pianist to improvise. For the horns I gave them the easier, more intuitive chord progression section, and divided it up so either Horn 1 or 2 gets to improvise and Horns 3 and 4 are playing backup along with the piano, and vice versa: a section for Horn 3 or 4 to improvise with Horns 1 and 2 doing the backing lines along with the piano. 

And the pianist gets the harder chord progression to improvise over, with all 4 horns doing backing lines. I also made it so the improvisation sections can be skipped entirely, and the piece will still make sense.

I got a big kick out of doing all of this intricacy like a chess game: if I make this move here in this part, what else needs to change? Does Horn 3 have a chance to breathe if they play this section and then have to play right away in the next section? Etc. Very satisfying, and I think it all came out well.

The piece becomes much stronger with these improvisation sections, and also has more justification, and the potential for even greater beauty because of what might get played during those sections! Co-creation indeed!