I don't really have a strong preference as to which Scheme implementation, web server or hosting platform to use. Mostly I'd just like to get a collaborator and I'm happy to use whatever (linux-based thing) they want to use :)

Up to now I mostly dabbled with the HTTP server side of some Schemes, but only really worked a bit more with Racket web server and with Kawa.

Also here Racket is feature-wise not far away from what the other large environments do offer, documentation-wise it's even better than what can usually be found if one can handle the academic touch of the Racket documentation. Also typed-racket and/or contracts can be helpful when working on something larger with multiple people involved, so Racket would be the safest bet from my point of view. It's just that there is always this small mental hurdle when switching between Scheme and Racket, that one might or might not get accustomed to over time.

Kawa has some shortcuts to quickly set up HTTP response handlers incl. some interesting template options plus it has access to the whole JVM environment, which one might consider an advantage or disadvantage. Also start-up times are still noticeably higher, but not bad enough to really hurt development.

In case we also plan to provide some elaborate browser-client functionality, Manuel Serrano's Hop and Hip-Hop would be very exotic and charming contenders. Hop (in it's application using Scheme for the client-side) is an incredibly cool piece of work (and I really like Bigloo for its very barebone touch), that's sadly not gaining any traction - maybe it just needs its killer application.

As for quite feature-complete Schemes which still feel very much Scheme-like, I like to work with Guile and Gauche and I would be fine working with any of those.

And maybe starting from somewhere with not to much HTTP infrastructure already available could be interesting in the context of the parallel ongoing endeavour for defining schemeweb - in case we feel brave enough to approach schemedoc as the first application of schemeweb ;)