I don't think there is any plan to replace or even supplement ELisp with Guile Scheme. Guile is a language, but it is also a compiler and runtime. The later two have been intentionally designed such that different language front ends can be written to use them. The plan here is to use a new Elisp front end for the Guile compiler and runtime to replace Emacs' custom Elisp interpreter. Extensions and internal implementations will still be written in Elisp.