That's great! :D
Since you asked for them, I Have a pair of ideas / suggestions ^_^
-A (configurable?) number of "undos" would be the most important to me. Ctrl+Z is my friend and I feel lost without him XD
-More export options would be great, like png sequence or animated gif.
-More "import" options to use animated textures for particles, not just spritesheet, but also png sequence or animated gif.
-More motion types, like linear, or following a path drawn by users, and also predeterminated paths, like circles, triangles, etc. would be great.
-I love the idea of using nodes for effects :D A new node for resizing (with option to choose between bilinear/nearest neighbour, maybe?) would be cool, that way the output would be more configurable without needing to change lots of parameters.
-Having separated effects for each emitter and being able to copy effects from one emitter to another one would probably be another nice feature.
And that's it, I've run out of ideas XD I hope you can use some of them ^^