Just tossing my slightly unrelated two cents in the matter by saying that I do see where you're coming from, G-Mixer, but honestly I'm glad the one-game-per-series restriction is in place. Sonic 3D and Sonic R both have some stellar tunes (dodgy singing in the SR tracks aside), and the leaked Sinic X-Treme songs are also pretty boss and I would love to hear them remixed, but even these days there's still something of a "the only noteable music in a sega made game or on a sega made console is the music from the sonic games" mentality in the pockets and corners of the Internet. It isn't as prevelant as it once was, but a Sega Saturn remix album with very little Sonic tunes on it might go a long way to getting people interested in soundtracks from the other great games on the Saturn.
Either way, it's not down to me what goes where. That's just what I think. This could honestly be a 20 track album dedicated to remixing nothing but Can You Feel The Sunshine and my only real complaint would be "pity nobody could find the original vocalist". Maybe.