pretty good!
well done on building this engine so far!
im going to state the obvious things that i feel need altering, not neccesarily bugs
firstly, obviously you need to make it so its either megadrive sonic, or gameboy sonic sprites, not both. (personally id prefer to see megadrive sonic just to differentiate it from ultimate flash sonic more ;) )
you could do with closing in the camera when sonic moves to the edges, its good that the camera isnt just fixed on sonic but it moves too much straight away, it would be good to have the camera slowly pushed that far to the sides when hes at maximum speed (if he has a maximum speed) but not when hes only just starting running.
gravity could do with decreasing a bit, so that he stays in the air at the peak for a bit longer
when you jump and change directions, the sprite changes to skidding
thats about all i could think of, i look forward to seeing this completed!