Shells, the thing you interact with in a terminal, parsing your input and executing programs on your behalf. Maybe you've grown tired of bash and zsh and yearn for something different.
Fish is probably the most popular (or at least loudest) alternative shell. With a focus on UX, colors and autosuggestions come by default, inspiring ports back to zsh (maybe bash?). Unfortunately not entirely POSIX compliant, so random scripts are likely to break.
elvish is further out than fish in terms of syntax, but it does mean things like lists are saner. Interesting things include the ability to pass data structures between elvish functions. Comes with builtin file manager.
Oil apparently sits somewhere between bash and python. Best approached from a (programming) language perspective. Barely documented.