I've written one specifically to solve if and looping: https://github.com/lmorg/murex (docs: https://murex.rocks)

Others I've not used are oil shell, which aims to be a better Bash. Elvish, which has a heck of a lot of similarities with my own project (and is arguably more mature too). There are REPL shells in LISP, Python and all sorts too.

So there are options out there if you want a $SHELL and happy to break from POSIX and/or bash compatibility.