First of all, this is an amazing piece of work.
The proportions of all characters are correct, also their poses.
Using your own style for Robotnik, Sonic and Sally gives it a unique touch I personaly like very much (especially the apparently mixture of classic and modern Sonic).
This absolutely reminds me to a common poster, a common setting, but still unfolding the typical impact and showing what is fact and happening in SatAM and presenting the main characters of it (mainly season 2).
While just being line art, it does nevertheless look finished and this is of course good!
A coloured version would be sure nice, but not necessary to improve this already very nice fan art.