Well, since you asked:

They said they split Solaris, the flame of hope, into two entities:
Iblis (The fire) and Mephiles (The shadows).
The revelation would be that Solaris wasn't the flame Elise blew out, Solaris was the whole room. And since Elise blew out the flame, the shadows covered the whole room instead of being only half the room.
Thus Sonic Nextgen only didn't happen because nobody split Solaris, because there's nothing to split, all of Solaris is simply Mephiles.
And thus after a long time (or in this case, the game where he'll return) Solaris will finally corrupt again (technically it would be the exact same time as Sonic Next Gen, only in a different scenario, basically the situation happens again only differently) and become Mephiles.
You'll see Mephiles as a Corrupted Sonic like he was a Corrupted Shadow, or a Corrupted Knuckles, etc. There are lots of different forms he'll get!
Sonic Mephiles!

Knuckles Mephiles!

The Gameplay would be like Sonic Unleashed Daystages, only a few changes:
B (Circle for PS3)'s function would be the "Special".
Special is a function unique for each character.
Sonic:
Tap Special while standing still = Somersault. Sonic will do a somersault which breaks obstacles, can be linked to the next move.
Hold Special while standing still = Spin Dash. Sonic will dash with his trademark move. You can jump allot further while Spindashing.
Tap Special while running = Somersault. Sonic will do a somersault which breaks obstacles, can be linked to the last move.
Hold Special while running = Roll. Sonic will roll and break obstacles. If there's a tight space to go over he'll slide under instead.
Tap Special while in the air = Stomp. Use it to stomp through weak grounds and discover a new path.
Hold Special while in the air = Roll. Sonic will roll and and break obstacles differently then if he jumps on it (he'll go through them instead of bouncing.)
Roll note: as long as Sonic is rolling, physics will be used to control it, like this:
http://www.youtube.com/watch?v=e3raSygEeLs
Only faster of course. This can be used to get to higher places by using ramps and momentum.
Tails:
Tap Special while standing still = Tail Spin. Tails will spin around with his tails. Can attack indefinitely if you keep tapping.
Hold Special while standing still = Tail Dash. Tails spins with his tails. Let go and he'll jump. Height of jump is determined compared to how much you charged.
Tap Special while running = Tail Spin. Tails will spin around with his tails. Can attack indefinitely if you keep tapping.
Hold Special while running = Tail Dash. Tails stops and spins with his tails. Let go and he'll jump. Height of jump is determined compared to how much you charged.
Tap Special while in the air = Extra Flight. If Tails is tired of his normal Flight he can use this to fly JUST a little bit more. He'll fly with Tails UNDER him. (Like in Sa3)
Hold Special while in the air = Extra Air Dash. Tails stays in place and spins his Tails in the way he usually runs. Let go and he'll Fly forward very quickly.
Tail Dash note: Tails is the only character who can break breakable walls directly above him, making him able to take lots of shortcuts no one can get to.
Knuckles:
Tap Special while standing still = Heat Attack. He'll dash to the most nearby enemy and punch it down.
Hold Special while standing still = Maximum Heat Attack. Charge and he'll dash like a drill to the most nearby enemy and destroys it. Can be used to destroy multiple enemies.
Tap Special while running = Foe Tossing Charge. Press it when there's an enemy and Knuckles will punch it way without slowing down.
Hold Special while running = Maximum Heat Attack. He'll stand still, charges and dashes like a drill to the most nearby enemy and destroys it. Can be used to destroy multiple enemies.
Tap Special while in the air = Hunter Dive. Knuckles will dive down. If there's an obstacle, he'll dig through it. If there's an enemy he'll grab it and punches it away.
Hold Special while in the air = Lightning Quake. Charges in the air and then when he hits the ground it'll create a shockwave.
Knuckles note: He gets through breakable walls just by walking into them, making him the fastest to get through breakable walls.
Lightning Quake note: This attack goes through breakable grounds and does a shockwave by the time it hits an unbreakable ground, making him the fastest to get through breakable grounds.
Shadow:
Tap Special while standing still = Chaos Spear. Throws Chaos Spear on enemy. Will be used on some levels to discover a path no one else can get.
Hold Special while standing still = Chaos Spear Barrage / Chaos Blast. Charge for one second and he'll throw 5 Chaos Spears at the same time. Charge for two seconds and he'll do Chaos Blast.
Tap Special while running = Chaos Spear. Throws Chaos Spear on enemy. Will be used on some levels to discover a path no one else can get.
Hold Special while running = Chaos Spear Barrage / Chaos Blast. Charge for one second and he'll throw 5 Chaos Spears at the same time. Charge for two seconds and he'll do Chaos Blast.
Tap Special while in the air = Chaos Spear. Throws Chaos Spear on enemy. Will be used on some levels to discover a path no one else can get.
Hold Special while in the air = Chaos Spear Barrage / Chaos Control. Charge for one second and he'll throw 5 Chaos Spears at the same time. Charge for two seconds and he'll do Chaos Control and destroys all enemies by teleporting to them.
Chaos Spear note: There will be enemies in unreachable places for other characters which Shadow can reach with Chaos Spear. Destroying them will make a door/passage/path/etc. be revealed which most only Shadow can take.
Chaos Control note: Can be used to get to an enemy unreachable for Homing Attack.