One structure I’ve enjoyed was the 64-bar Challenge, which developed on the Ninja Tune Forum about a decade ago.
They gave a BPM and participants had 64 bars plus two bars either side for mixing. Often these would be a beatless intro, then a break for the outro.
I like the idea of layering results though, as this would get a richer result.
Not sure if it needs an agreed key or if a skilled mixer could bring different keys together?