You might look at UIOP and particularly UIOP:run-program. You could probably build a DSL (like bash) on top of these. Since they are part of ASDF, they are included in most Lisps.

I would think that having a lisp repl attached to a simple gui file browser would be nice. Having lambdas and recursion when doing "bash scripting" would be very nice.

I don't really care about immutable file systems, personally. I think immutability is more cumbersome than making occasional backups. Are you trying to replace bash or Unix?

I haven't tried any of this; I think you'll have the most luck if you start playing around with UIOP and build something simple off of that to start. You can make enough of a prototype to see if it feels worthwhile to use. Play around with your own pipe idioms. Limiting your scope to just fixing Bash seems like a good idea, since projects like this tend to spiral out into limbo. Maybe pick a few (three?) of the most important things you want to fix about bash and just do that? That might be enough to see if it's a good idea.