u/Creatresscreature

For my part, I hope it's option 1. To have Metal directly betray Eggman on screen, demonstrating that he's acquired free will and has a goal that no longer aligns with Eggman's - that would be the most powerful way to display it in a movie.

The one thing I could see being a problem is that we already had an on-screen betrayal with Gerald, but I do think this can be a very different beast. When Gerald betrayed him, Eggman fought back, and won in the end (but with help). Metal Sonic betraying him, especially if it happens as he upgrades and gains power, would leave Eggman completely helpless, particularly if Metal then takes over his robot army. For once, Ivo would need to fully ask for help from Sonic and friends, because his machines are everything to him, and he won't feel like he's much without them.

Additionally, if Metal gains a voice as part of his upgrades, possibly along with becoming Neo Metal Sonic, a scene like this would be a great place for him to speak for the first time. Although any scene vs Sonic as he upgrades could also work pretty well.

But those are my thoughts - what are yours? If you prefer the other options or like 1 for different reasons, please explain below.

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The Sonic games have used numerous methods of Time Travel over the years, but, despite Sonic 4 being heavily rumored to involve time travel in its plot, we have no idea how the various characters will actually use it. I think there are a few main options we could see:

1. Chaos Emeralds/Chaos Energy

For at least some instances of time travel, I think we're pretty guaranteed to have this be the cause. There could be some specific conditions for it to be possible like in Sonic 06, or it could be a little more ambiguous. The probable plot we'll have is that Eggman took over the future, and the likely way he got there is through some rift caused by Super Shadow and the Ark explosion. Since Shadow is also rumored to have a side plot in the past, this could also be how he got there, since the post-credit scene doesn't have anything specifying a particular time period when he stands up (although he could just as easily travel back in time at the beginning of the movie, using any of these methods). This one is more of a guess, but if Metal Sonic is built with Sonic's Chaos Energy-powered quill, then he might be able to use the raw power in it to travel through time and space without any help, and that could make him all the more threatening. Also, if the rumored sections in the past contain Chaos Emeralds, obtaining them could also be how the characters in those time periods return to the present or future. However, several of these scenarios might also be accomplished with...

2. Time Machines

In the Sonic Rivals series, which introduced the current iteration of Silver to the rest of the Sonic cast, the time travel is mostly accomplished by time machines that were invented in the future, probably by Eggman Nega. This could be the case with Sonic 4 as well, except, of course, they would be built by Eggman himself this time (and maybe invented by him too). How much of the future tech he takes advantage of will be things of his own making, and how much will just be what's available in that future, all remains to be seen, but I can definitely see him wanting to "mechanize" the spacetime-warping powers of Chaos Energy (as he basically tried to do in Sonic 2). This is another way Metal Sonic could travel through time (it could even be built into his body somewhere), but whether or not the other characters would use some kind of pocket time machines to travel through time is another question. And of course, that doesn't rule out the possibility of...

3. Both/A Combination of These Two Ideas

Of course, both of these are entirely possible, especially if any time machines are powered by Chaos Energy or Chaos Emeralds themselves. In fact, it wouldn't surprise me at all if Eggman builds a giant time machine or some other super powerful spacetime-warping machine in the future that's powered by one last Chaos Emerald for everyone to get (and this could also make for a great adaptation of a final Eggmanland-style battle too!). Metal Sonic could also be able to time travel with a combination of the Chaos Energy in Sonic's quill powering some specific mechanism he has. We could also get a mixture of characters in the future or even the present using machines to travel through time and space (probably still ring portals for space though), and then in the past, having to get out by finding Chaos Emeralds alone.

There are also some other notable time travel methods used in the games, but I very much doubt we would see them in this movie. In particular: the Time Stones and the spacetime powers of Mephiles/Solaris. I don't think we'd see the former because it would be overload for the movies to introduce another set of MacGuffins, and the latter we wouldn't see because he just hasn't been introduced in the movies yet. But what do you guys think? The trailer should be coming soon - we might get answers to this question when it comes out.