The proposal itself sounds like something that would be worth doing, if only to allow enabling “experimental” technologies to “prove” themselves before being included in Fedora “proper”. However, I see some potential issues (or at least questions) about how the Sandbox mechanism would actually work:
For example, where would sandboxed experiments that affect alternative (or additional) versions of packages / images / deliverables get developed (where would the sources live?) and built? CentOS Stream has CBS for this, where CentOS SIGs can do something similar, but Fedora does not (yet?) have an equivalent “second koji”. Who would provide and maintain this additional infrastructure?
On the other hand, some Sandbox projects might require changing some Packaging Guidelines and / or FESCo policies. One example that I could imagine would be a Sandbox project to provide alternative Kernel packages that enable some experimental features or include non-upstream patches, or even kmod packages to ship kernel modules that are not enabled in the “stock” Fedora kernel. Both would (currently) violate the Packaging Guidelines, which only allow for one kernel package and no kmods external to the kernel. Do you expect to update docs / policies and guidelines to account for Sandbox projects like when needed?