Aside from the 68000, and perhaps the similar resolution and number of colors, the Genesis and Amiga are very different. Sonic does a great job of taking advantage of the former's abilities, which include two separate character-based BG layers. Having the multiple, independently scrolling layers would be problematic on OCS/ECS because if you use the hardware dual playfield mode you're limitted to 3-bits per pixel per layer (8 colors). I think you could make a great Sonic clone that way but the look would be inferior (probably OK on AGA though with 2x 4-bpp layers). Alternatively you could use 5 or more bitplanes and only fill in the edges of the screen between frames as you scroll. Parallax effects would have to be mostly abandoned, so again it wouldn't look as nice as the original. In any case, please don't use double frame buffers and settle for a crappy frame rate.
As for the copyright infringement aspect, it should be noted that there are various Sega related fan games floating around and AFAIK they have never complained. Someone I know that is involved with one such project said a Sega employee OK'd it (unofficially).
