One option is to have both Lua and Elisp, so that packages can be slowly migrated. Lua is small (so it doesn't incur a huge size cost to have 2 engines), fast (one reason why they want to switch, according to the author), and has a significantly large community. It also has coroutines, which is not what people dream of when people talk about concurrency but is probably good enough for a text editor (and avoids most sync problems). Furthermore, with this "dual-engine" strategy it would be a bad idea to choose a language similar to ELisp: it would cause a lot of confusion.

But I guess that Emacs on Lua wouldn't be Emacs anymore. That's the main objection.