I think mIRC should support "mass" actions like mass ban, kick, voice, devoice, op, deop, etc. I had to write a bunch of aliases / scripts to do it...
I'd also like to be able to "arrange" my channel windows by clicking on it and dragging it or something. So, that way, I can move a channel's "minimized" window around the bar.
Also, the ability to use your own browser, I also had to write a script to use mozilla for URLS instead of IE.
Also, I think it would be good to be able to "bind" keys to commands and edit the current mIRC hotkeys, like, you'd be able to change control + n to not only go to the next channel window, but also query windows and stuff.
Another cool thing would be auto-rejoin channel on unban, like, it'd constantly keep trying to "join" but wouldn't tell you in the status window, until you finally join.
I also think that it would be a good feature to be able to "break up" the userlist into catergories like :
It'd also be cool if mIRC could collect channel stats and generate an HTML file based on the various stats collections options (you could decide what kind of stuff you wanted to collect, how many things to list, etc)