I use /fill a lot. What bothers me is that I have to constantly do /fill with lava, or water. I know water can be made in 2x2 for infinite source, but sometimes I can't do that. My suggestion is that /fill should be unlitimed. Meaning you do /fill once, and to emtpy it, you do /unfill or something. Also, to change the liquid, you shouldn't empty the bucket first, and just type /fill lava (when you have a water bucket) and it changes to lava.
