It'd be amazing if it crystallizes in a few years. Improving the old parts of Emacs, especially GUI code and low level things is a must. Rust seems like an ideal replacement for C. Great performance, much better safety guarantees.
Equally important is perhaps improving elisp concurrency. The jury is still out on whether this will happen by migrating to Guile Scheme [1].
[1] https://www.reddit.com/r/emacs/comments/4zttlt/guileemacs_st...