I don't know how viable this would be, but my suggestion is that there be a list of words that can be tab-completed into the editbox. Currently the nicks and #chans can be completed, but it may be useful to be able to complete other things as well, such as websites, common commands, etc. The exact things that are allowed in the list (spaces, special characters, etc) would depend on what would work in mIRC. If possible, multiple words could be included in a single item, like typing "//mo<tab>" could return "//mode" and then "//mode $me" etc.
Also, there could be options for the completion list. Either for each item or for the whole list, the user could choose in what order similar items are displayed (by moving them up or down the list), and also whether they are displayed before or after the regular items (nicks and #chans). An option to enable/disable each item as well as to enable/disable the whole custom completion system, or enable it only in certain windows/channels.