I'm not sure such thing would be very doable with what currently exists. McCLIM offers counterparts to miscellaneous Genera utilities (the command listener, debugger, inspector, etc), and CLIM itself was Symbolics' attempt at a portable, standardized Dynamic Windows; running them in a single image, and treating the command listener (with a few commands to replace UNIX shell miscellanea and whatnot) as your shell, and whatnot could bring you an inch closer, but there's barely anything written for CLIM, or cooperating with the premise, and even what currently exists doesn't fit together as cohesively as the Lisp systems did (though there's definitely work being done).
Another option I see could be Emacs with EXWM, and trying to do everything with Elisp utilities. Emacs mostly fits the bill for a single address space, image-based, largely introspectable environment, and there's a surprisingly large amount of Elisp utilities for all sorts of tasks; but compared to Genera it's still kind of pathetic.
Much of what makes Genera (and the Lisp Machine systems in general) magical isn't really skin-deep behavior, but rather a consequence of fundamental system design characteristics (single address space, image-based, introspectability and openness, the pervasive presentation-based toolkit, etc) that aren't really possible to replicate by haphazardly throwing together a bunch of loosely-coupled hacks on top of UNIX. You can try though, and I would be interested if anybody gets anywhere significant.