I would use Python as the extension language since it is omnipresent. ELisp has lots of terrible misfeatures that makes building large software in it difficult.

I would use Python as the extension language since it is omnipresent. ELisp has lots of terrible misfeatures that makes building large software in it difficult. I would also add better support for the services model. E.g., rather than adding spell-checking and syntax-highlighting modes you could defer those tasks to LSP servers.