If it's reasonably well-detailed, I would suggest
GaiaOnline's comic creators section (under art discussion)...
I would suggest making sure that it has few cliches, doesn't involve Japanese places/names (unless there is a good reason), and no Mary Sue's first, and then making sure that the idea includes a reasonably detailed synopsis of the plot, and descriptions of the characters and locations.
You could even post it here....
FR