Well, I was thinking and there were some things that I thought seemed to suggest this. I've always thought Mephiles looked a lot like Metal Sonic (just look at their faces), and then I remembered two more things. Firstly, Metal Sonic was supposed to be in the game at one point; leftover data shows he had his own episode, so he may have played an important role. Secondly, this is a little bit weak, but Mephiles' arc heavily involved time travel, something we could associate with Metal Sonic given his role in Sonic CD. Maybe I am talking nonsense, but I did want to point this out. It could have been very, very early in development, before Sonic Team came up with Solaris and further developed Soleanna's lore.