Neovim will have Lua (and LuaJIT) baked-in too, so Lua will always be there.
In my opinion initially there might be many different languages used by developers as they try out the new versatile plugin infrastructure. But after a while only a few languages should emerge as the dominant ones, because developers have the desire to help as many people as they can by making their plugins easy to be deployed and configured. I would guess Vimscript and Lua(both baked in), Python(nowadays it's practically ubiquitous) are the more popular choices. Personally I like Go (fast, easy concurrency, and single executable deployment).
In my opinion initially there might be many different languages used by developers as they try out the new versatile plugin infrastructure. But after a while only a few languages should emerge as the dominant ones, because developers have the desire to help as many people as they can by making their plugins easy to be deployed and configured. I would guess Vimscript and Lua(both baked in), Python(nowadays it's practically ubiquitous) are the more popular choices. Personally I like Go (fast, easy concurrency, and single executable deployment).