I'd rather GT7 have a completely new original soundtrack after GT Sport, just so it feels completely fresh once you first boot up the game. I love the classic songs, but I personally want something new.
With that said, I think it would be an awesome idea if menu soundeffects and soundtracks from past games could be rewards for certain events, then enabled. This would give players the choice to play GT7 with (for example) GT4 menu sound effects, and a GT5 soundtrack. I think it would work best as endgame rewards though.