Bit weird I know, but what do we think the story of Sonic 2017 will be? I know in the end, it'll be straight forward, but going by the trailer, it seems to me there's a bit more in it this time. As me and Barry previously discussed, it seems like they're going for the soft reboot like Days of Future Past with Robotnik messing with time.
I wonder if modern Sonic will have to go back and stop modern Robotnik and we'll see the previous adventures wiped away in some way. Saying that, there doesn't seem to actually be a timeline anyway.
I don't think they will go that far, but I do think they are going for that "darkest timeline" sort of story where shit hits the fan, Sonic calls on his friends from both past and present, and they take on Eggman. I feel that if time travel does play a part, it will mainly be when Classic Sonic is called upon and perhaps a good/bad future mechanic like Sonic CD.
I dont think theres a real timeline for Sonic games... They all exist in their own little bubbles, some are sequals like Sonic 1 2 and 3k... Adventure 1 and 2... Colours and Lost World... But mostly they seem unconnected.
SEGA and Sonic Team do have a bible that they reference, so there is a loose continuity there. From what I've gathered, the Genesis/Mega Drive originals are forever the backstory and that, like Sonic X, there exists two worlds (humans and animals) that have since merged. This is why Iizuka said in an interview back in 2010 that some games take place in the human world (Adeventure, Adventure 2, Unleashed) and some in the animal world (Heroes, Colors, Generations) however in a recent interview with Aaron Webber he clarified this a little by saying that these two worlds are two planets. So there was our Earth and Sonic's Earth but they have since merged.
Personally, for the sake of sanity, Aaron likes to think of each era and storyline as its own contained world. I agree with him on this. I mean, yeah, retroactively Sonic Team's bible could say that the Genesis/MD storyline was on Sonic's world and Eggman crossed worlds to get there, and Sonic and friends traveled to the Human world in Sonic Adventure either through a portal or a dimensional shift that merged the two. I think Sonic Team's new explanation was made just to explain the shift between the 16-bit era and the Dreamcast era.
However, in reality (like, our reality where they're just games) I think SEGA and Sonic Team was never quite certain on an overarching explanation for where the games are set and why there are humans and talking animals.
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I categorize the games this way:
Genesis/Mega Drive Era: Sonic, Sonic CD, Sonic 2, Sonic 3 & Knuckles
Dreamcast/Sixth Generation Hardware Era: Sonic Adventure, Sonic Adventure 2, Sonic Heroes, Shadow the Hedgehog
Failed Reboot: Sonic 2006 (events of Sonic 1 through Shadow the Hedgehog serve as backstory, but series was starting fresh with new designs and direction)
Modern Era: Sonic Unleashed, Sonic Colors, Sonic Generations (which brought Sonic '06 into the Modern Era by referencing it), Sonic Lost World, Project Sonic 2017
I would also say that there was another failed era, which was Sonic the Hedgehog 4 Episodes 1&2 which tried to expand on the Genesis/Mega Drive Era. I totally believe that had Sonic 4 been a success, we would have seen Episode 3 wrap Sonic 4 up and a Sonic 5, Sonic 6, etc. Not a reboot, but an expansion. Sonic Mania is now SEGA's second attempt to go back in time and expand on the Genesis/Mega Drive era, basically a Sonic 4 redo.