Beat the game! Clocked about 35 hours doing 100% completion, and added a little more onto that to do some time attacking. More thoughts:
There are really two main gameplay cycles on the overworld areas. The first is the one that's heavily derivative of Shadow of the Colossus, where you take on large bosses and defeat them to get gear tokens. Those gear tokens then get redeemed to access cyberspace levels, which are basically this game's take on special stages, interpreted through the same ideas as Sonic Colors' Sonic Simulator -- which is to say that nearly every level is copied from a previous source, a combination of several SA2 full stages and portions of those from Unleashed and Generations. These then grant you keys to unlock the chaos emeralds, which will eventually lead you to the boss. It makes a lot more sense than Sonic Simulator, though; the layer of indirection between cyberspace giving you keys which you collect enough of to unlock an emerald may start sounding like Donkey Kong 64's tons of different in-game currencies but there wasn't another good way to do it because of the degree to which the use of the emeralds is part of the game's plot. Sonic Colors' placement of the emeralds doesn't hold up well to narrative scrutiny (how did they get into...an arcade game? that's owned by Eggman? where he isn't taking advantage of their power to stop Sonic??) but Frontiers doesn't have the luxury of handwaving it away; "why are the emeralds where they are" is one of the core questions the game's plot is concerned with answering.
Speaking of cyberspace, though,
Sonic Retro's page on Cyber Space has a mostly complete list of sources, but it misses a few references. In particular: 1-3 is based on the 2D segments from Windmill Isle Act 2, 2-3 is the 2D segments from (Unleashed) Rooftop Run (at least up until the drop through the tower), 3-4 is based on part of Windmill Isle Act 3 (although, weirdly, the second part of
that stage doesn't show up here but in a transition area between islands; the rest more resembles some 2D sections of Lost World's Tropical Coast 3 and Lava Mountain 2), and 3-5 is based on Savannah Citadel Act 2. Others are more debatable: 4-2 is probably supposed to be the 3D segments of Speed Highway Act 2 (the wall-run is unmistakable) but seems like a pretty loose interpretation, and 4-3 is probably 3D segments of Seaside Hill Act 2 (I see a few split paths that look similar and what looks like the transition point to the kart segment, but there have been a lot of changes made including extra rails and ziplines). 4-7
might be the first half of Radical Highway (the second part, focused on the bridge section, occurred much earlier in the game) and 4-8 seems to my eye like it's built from other parts of Seaside Hill Act 2, but both have been much more heavily edited and altered from the source material than nearly every other stage. 4-5 and 4-9, oddly, don't have any obvious antecedents (though 4-9 can't help but resemble the intro to City Escape; I'd also seen a youtube comment suggesting that 4-5 might be based on Cool Edge DLC levels but it's an especially loose adaptation if so, and the only cyberspace stage based on a DLC level if we took that for granted).
Before getting into the second gameplay cycle, I should note that while comparisons to Breath of the Wild are overstated -- I think most of the ways in which this game resemble that one are ones in which BOTW broke from aspects of series tradition and instead took inspiration from Shadow of the Colossus, so that's a more obvious, more direct, and frankly more salient comparison -- there is one thing that feels directly inspired by it. The map-visibility tower climbs and korok collection mini-challenges are combined into the puzzle/technical-challenges that reveal map completion in this game. These aren't terribly complicated for the most part -- like the korok challenges, they tend to be pretty basic skills tests and usually only take one or two tries to complete properly -- but revealing the map is the first step to wider game completion. My understanding that even
this is more like BOTW's influences (i.e., Far Cry) than directly anything from BOTW as the map reveal doesn't simply show extra pieces of territory but also is key to revealing all the stuff to be interacted with in those areas, like the aforementioned titans and cyberspace entrances, as well as character tokens and interactions, which are the core of the second gameplay loop.
This is apparently where my experience differs heavily from just about everyone else's. This challenge is extremely open-ended and I loved just about every dang minute of it. I also liked the way that it developed across the game. In the first island? Memory tokens are easy to find, usually kept near the ground, and so it's easy to wander around and find the structures leading to them. This early in the game you're going to be more focused on figuring out the titans and unlocking emeralds; the token requirements stay low. The second island spreads them out more, makes the different areas a little harder to get to, and definitely has you spending a lot more time tracing out the structures to figure out where they begin and end. And this trains you to start thinking more about the structures in the other two main islands (the third and fifth, as the fourth breaks this pace, more story-focused), and you spend more time hacking and trying to get around the structures. Other people apparently (based on the retronauts episode, which I have listened to completely! and I'm certainly convinced nobody on that podcast beat Sonic Unleashed) don't like the third island because there are a lot more segments where the camera perspective switches to 2D. And to be fair, I've been fairly critical of those 2D segments (they're a key part of why Colors landed worse for me) because they screw up the handling and make Sonic slower. When they've eliminated an entire potential source of controller mishaps by getting rid of a dimension, they should be making Sonic able to handle looser, not tighter! He genuinely feels too slow in 2D, especially in this game! But what's great about that island is that you can nearly always ignore it. Rather than starting from the beginning of any of these segments, you can frequently start from the end, or chain one memory token grab into multiple, because they tend to be close together and it's much easier to recognize the telltale signs of where one of these micro-levels end. And what I love about this is that all of this reinforces the storyline up until this point. In the first island, the story is mostly about exploring the island and trying to figure out what's going on. In the second, you start thinking about its purposes and how things got put where they are. By the third, you're actively seeking to commandeer parts of it, and by the final island you're working to break it over your knee. And what I love is that is also what I was doing while I was playing the game. People point out that the story this time is a little weightier than it has been, but what I really appreciate is that the actual moment-to-moment interactions with the game help to reinforce it; there was a lot of thought put into the way the game actually feeds into it.
That's not to say this loop is above criticism -- the low draw distance is disappointing and when you can configure the level of aerial deceleration Sonic has but not that graphical setting, the game has an obvious flaw in that gameplay loop. It's too easy to lose track of where the structures begin and end simply because the game does not provide access to that information depending on your position and camera angles.
It does feel a lot like Mario Odyssey, though, because tokens are about as readily available as the moons were, your intuition about where they are is probably correct, but actually reaching them may require an indirect route across a number of gimmicked areas if you don't try to work backwards.
Moments outside of these loops, however, tend to be pretty crap. The laser puzzle isn't terribly hard but it is a very unintuitive thing to control by the standards of Sonic (even by the standards of the rest of *this* game, where most of the abstract challenges tend to be things like 'press the dodge button a bunch of times'). The orb-moving crane is so simplistic as to be laughable. The pinball takes too long on the hardest difficulty.
While I don't think this is as deeply inspired by BOTW as some people make it out to be, it's funny that this game feels like it's built out of a pretty strong list of influential Japanese games from the last let's say 15-20 years. If you photoshopped Sonic into a screenshot of Shadow of the Colossus, you could probably convince people who have played this game you're showing them a screenshot. There's also a pretty significant debt to Nier, I'd argue, both in tone and theme, though this is understandably more friendly to children. Mario Odyssey I already mentioned. Hell, even Radiant Silvergun and Ikaruga get surprisingly direct references. You'd expect, given that, that there'd be something somewhere that feels like Katamari, but surprisingly I didn't get much out of it. The game still manages to feel like a coherent whole regardless, not just a patched-together list of influences. That's the benefit of an original, emotionally authentic story I guess?
Which is not to say that I loved everything about the story. The need to bring in references to all the other games gets exhausting. It's practically on the level of Family Guy. Hell, you could make a Family Guy style joke out of all the callbacks by capturing footage of these "remember that time when" bits and then cutting to each of the respective games glitching out somewhere, and god do I hope someone has done this.
I also think it's weird that they've tried to explain what the story of the chaos emeralds is when we've already had that established in Sonic Origins. All those emeralds? They held the world in place and kept that Dark Gaia fellow from escaping and causing havok on the world. That's what these emeralds do, sort of the opposite of the master emerald which keeps a different continent in the air. That was consistent with Sonic Advance 3, where the emeralds were used to do something similar. Now they're, uh, from space. Somehow. But the master emerald isn't, that was there the whole time. Weird, inconsistent, and kind of jarring that they'd make all these superficial references to, like, everything, but then forget that.
But who knows. About an hour and a half ago they dropped a new story update and maybe that's got something to do with it. The side characters are, apparently, playable.