<3 <3 <3 The only symbol I've wanted that apparently doesn't have a sequence is for pi. And I know you can add sequences in Linux, but haven't been bothered to figure out if/how with wincompose. heh.
Have fun! Oh, and one bit that took me a long time to find but I use all the time; ① - numbers in circles, a rare three-character sequence - parentheses surrounding a number i.e. (1) :) (numbers ① through ㊿ exist)