While my kit is very quickly nearing a state I can feel quite satisfied with (though I wouldn't say completion since that is hard to define with gear), there are of course some non-essential things that would be cool to hav for once-in-a-while use.
Reversal ring for Canon EOS cameras.
Or more preferrably:
Neutral Density filter, ~8X, such as this. (Needs to be 77mm.)
Flash gel set.
Update: since I got a 40D, I could also use a Super precision matte focus screen! But I guess that would be hard to get on a short notice.
Then there are other things that would be cool.