The currency symbol and logo/symbol don't have to be the same.
A symbol is a symbol without text. It should embody the ideals of the movement. The logo should have text at first, so we can build our brand. It should incorporate the symbol somehow.
For a symbol, I think tomoe is good, but I am playing around with some other ideas, like describing the POI algorithm graphically in the symbol. Here is one idea (just a sketch at this point; this shows that different accounts have different importances):
Now that is unique. It looks like a crop circle!
https://en.wikipedia.org/wiki/Crop_circle
http://www.cropcirclelogos.com/
But... it is too complex and at a distance it is too similar to the logos of Ripple and Quark.
I thought the same about ripple and quark, so I am still playing around with this idea: http://forum.nemcoin.com/index.php?topic=2228.msg2936#msg2936