Well, there was a possibility of a Saturn 2 console (years before Dreamcast) that never happened. It's much less 'solid' than M2, in that there is no confirmation that it existed. However there were many reports & rumors of a Saturn 2, based around a Real3D chipset from Lockheed Martin, the providers of the 3D graphics tech in Sega's Model 2 and Model 3 arcade boards.
There were many articles on Saturn 2 in print and on the web back in the mid 90s, but this was probably the biggest and best:
Next Generation - November 1995
http://img80.imageshack.us/i/saturn2lmc ... 806js.jpg/
http://img122.imageshack.us/i/saturn2lm ... 522rk.jpg/
http://img122.imageshack.us/i/saturn2lm ... 140sq.jpg/
If Saturn 2 had a PowerPC 603 CPU and a Real3D/100 graphic chipset, it would've surpassed M2 in visual performance, by around 2x
Only the 3DO-designed MX (M2.5) chipset would've been more powerful.
The Saturn 2 that I envision for release in 1996-1997 looks something like this (the Hi Saturn Navi):

It has this for a graphics subsystem:

And produces images like this:

It's basicly SEGA's answer to the 3DO-designed, Matsushita manufactured M2:

M2 graphics:


Except that this Saturn is somewhat more powerful.