For the transitions, couldn't we ask each composer to start and finish their piece with 2 or 3 bars of the tonic chord (on sustaining pads such as strings or synth voices) of the key they're selected to start and finish in? Then immediate cross-fading between sections would be easier and smoother.
Or have you already sussed out how you're gonna do this?
It is already discussed some - but to add... It would be more interesting to have more than just a sustain. perhaps some instruments outlining the tonic, some percussion, something that seems like the piece is still going instead of stopping and pickup after a crossfade...