Yep it should be a nice idea :P
Never used the Favorites/Chan list/Tile/Cascade icons and 'd like to be replaced by some customized one, to /run an external program or a script for e.g.
By the way, Large icons and brief text options too (think on user's accessibility. I'm sure some people with disabilities are using mIRC).
If this can be done, why not introducing some sort of "themes", where users should be able to customize everything?