IPython already makes for a pretty good bash/zsh/etc replacement for many things. You can get a GUI using the IPython notebooks as well.
While I haven't made the switch completely yet, I always keep an IPython session around in an extra tab, since it is very often just much simpler to do something there than in bash.
I actually meant a terminal tab (I'm using Tilda as terminal emulator). I'm only really using the web-based notebook interface when I need to display graphics for something.