New Theory on the Iblis Trigger:
Ok, this is just a suggestion as to what the story might be:
Somewhere,Sometime (black comet maybe?) something got inside of Sonic. Over time, it started to take control of him. Meanwhile, Shadow is doing his thing when he notices a strange Sonic. Which is Sonic in Iblis's First Form perhaps? Those creatures look ALOT like Black Aliens, so its possible Black Doom could be in on this. Anyway, similar to Chaos, his power feeds on Chaos Emeralds, and Shadow is defeated. Silver is in the future, where Sonic got all the chaos emeralds, and is no longer Sonic, but Iblis (the Fire Creature). Silver cannot defeat the Iblis, so he goes back to the past to find the 'Iblis Trigger' and find Sonic. Perhaps to cure him, or maybe to destroy him. So he goes to the past and after maybe years of searching, he spots Sonic. "I've finally found it, the Iblis Trigger.". Thus he goes to stop Sonic.
Meanwhile, Eggman is raiding Soleonna. "I've come here to obtain the secret of the Flames of Disaster, and to take the miracle gems that are the key to this secret, Ah. The Chaos Emerald!"
This 'Secret' is how Sonic becomes The Iblis.
Meanwhile, Sonic goes to rescue the Princess, but is stopped by Silver.
Eggman gets a hold of the chaos emeralds, and (not sure how yet) obtains the secret. Eggman tricks Silver to capturing Sonic alive. Sonic is brought to Eggman, who gives him the chaos emerald...as the creature takes hold inside of him.
Eggman: Time to demonstrate your power.
Shadow finds the 'Iblis' Sonic (Perhaps he was warned by Silver ???) And is attacked by him. The Iblis takes Shadow's Chaos Emerald and becomes even more powerful, then returns to Eggman.
After a while, he gets all the chaos emeralds as Sonic reaches his final form...The Fire Demon you see in the trailers.
Silver: This is what i was trying to prevent...
Silver realises he is the cause of the bad future, and goes to stop it. Perhaps he is aided by Shadow, or maybe even the Past form of Sonic.