I recently saw
this feature request on github, and thought it deserved some more attention. Tab completion is something I miss a lot, considering that it takes quite a while to type out commands (such as /grant singleplayer all). This would entail giving a way of adding custom completions to user defined commands. This could be in the form of a function which given a string to complete, returns a list of possible completions. Any ideas on this would be welcome.