Emacs 29+ will have optional tree-sitter support out of the box. A lot of modes are reimplemented to use a dedicated shared library for faster parsing and other features. E.g. elixir-ts-mode vs elixir-mode. This might not be as beneficial for Fennel, because s-expressions are easier to parse, but we might be able to implement some highlighting features, such as highlighting where only in case or match, and other things like that, without sacrificing performance or writing convoluted regular expressions. In addition to that, Emacs 30+ sets forward-sexp to a tree sitter-backed function, thus making it possible to create a universal structural editing interface. While again, not a problem for s-expression-based language, obliging to a universal interface may be more future-proof.
You can assign me to that unless there's anyone else who wants to do this before I get to it.
We already have a grammar: https://github.com/TravonteD/tree-sitter-fennel but it might need some adjustments down the line.
It sounds like if someone is already working on this in a 3rd-party repo it's better to track this there.