These are just some of my theories for Sonic Forces.
Sources: Infinite's Theme, the Enter Infinite trailer (links in the comments ---->)

(This is just a funny theory; I don't expect it to be real...or do I? Eh, probably not.) Infinite is/was an extremely overpowered OC who is rebelling for some reason

Infinite is some kind of digital being (basis of theory: in the Enter Infinite video, there are random glitchy spots that appear and disappear around him; I haven't seen anyone anywhere mention or point this out), or, what we're shown is just a digital projection of him.

The whole game (or at least most of it) happens in a virtual reality. (somewhat-supporting evidence: the lines in Infinite's theme that say "This is an illusion, open up your eyes" and "Take a look around and tell me what you really see")

In the context of the illusion line of Infinite's theme, the line right before says "When there's no one left to carry on", which could mean that someone (or everyone) appears to be dead or something, but really aren't. This can support the virtual reality theory, but not exclusively (it can be worked out in other scenarios as well).

The line "I can live a lie, and that's the difference in you and me" appears to support the digital projection theory, though it could be talking about that he covers up his past.

Infinite hails from Classic Sonic's dimension. (LATER EDIT/ADDITION)(Basis of theory: in the Enter Infinite trailer, near the end when he's talking to modern Sonic, he says "The LITTLE blue savior", and modern Sonic obviously isn't little (when compared to Infinite, anyway))

If the previous is true, then if the custom character (henceforth known as the OC) knows anything about Infinite (which they most likely do), then they most likely come from the same dimension as Infinite.

Infinite and the OC both come from a completely different dimension, where Infinite has gotten rid of Sonic for good, and the OC is his biggest roadblock now. Or was, until whatever happened happened and they both ended up in the prime dimension.

(Question: If Infinite is a being that can travel between dimensions, and if he is from a different dimension, then why is he just now making an appearance in the prime zone?)

In connection with the "When there's no one left to carry on" line, instead of everyone appearing to be dead, what if, one by one, Infinite captured and locked them all away in a pocket dimension or something?

(vv LATER EDIT/ADDITIONs vv)

The first few lines of Infinite's theme that say "I'm the tallest of mountains, I am the roughest of waves, I'm the toughest of terrors, I am the darkest of days"; what if they are referring to Zavok, Chaos, Metal Sonic and Shadow, respectively? What if Infinite is manifesting himself as those characters, or projecting a digital solid-light image of them, and those characters we're shown aren't actually the real Zavok, Chaos, Metal Sonic and Shadow? ...all of whom have been some of Sonic's toughest foes! Hmmm...

Infinite is a Syndrome (from The Incredibles)-type character; he just wanted to help the good guys/bad guys, but was always pushed away, hence the "I was born in this pain, it only hurts if you let it" and "This pain persists" lines; so he rose up on his own and became the baddest of the bad, hence the "I won't stop until they know my name" and "So take what's mine..." lines, and possibly the "Only scars remain of who I was" and "I can live a lie" lines.

This is just a wild, crazy theory that I HIGHLY DOUBT will be true, but Infinite could be an alternate version of Sonic who turned evil most likely because of the above-mentioned theory. (But how would his appearance be almost completely changed?)

As an alternate to the "The little blue savior" line theory mentioned above, instead of Infinite coming from classic Sonic's dimension, he could have known modern Sonic as a little kid.

This is more of an observation that supports a theory rather than an actual theory, but in the Enter Infinite trailer, after Sonic boost dashes and Infinite sidesteps him, after Sonic says "Huh?", you can see that Shadow's pupils are more like slits (the green circled area in the picture below) rather than the oval shape they usually are, and there's no shine circle like there is in the E3 trailer. So, either Shadow is being controlled somehow, or, like I suggested a few theories up, that's not Shadow.

I'm still trying to figure out what Infinite meant by "But what's this I smell?" in the Enter Infinite trailer, the second half of the "The little blue savior" sentence. There's something strange about that line, and I want to try to figure it out. Am I wondering about nothing, though? Or will that be an important part of the story? I guess only time will tell.

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8/8/17

Edited/Updated 8/19/17