Let's just get right into it.
"Why Mewtwo VS Shadow?"
Where to start? Okay, so the matchup's obviously been around for a long time, and happened a while ago as well. Quite frankly, it's one of the best matches the show has ever done. The characters have so much in common, there's a good basis for banter, there's a lot of emotional depth to explore there, their powers go well together in what can make for a VERY flashy spectacle...
...and the episode doesn't reflect any of this. At all.
Season 3 was rough. The show had just changed to a yearly schedule and was pretty obviously experiencing growing pains as a result. Torrian smoothed things over with his insane (read: unhealthy) work ethic, yet everything else was just suffering. But even for the standards of this often looked down upon era, this episode SUCKS. It's really bad.
First, the analyses are a big bunch of nothing. Not particularly funny, not a whole lot of meat to 'em, perfectly forgettable. Then there's everything else. This episode had a lot of unfortunate circumstances in its way. Neither vocal performance is spectacular, but Shadow's is particularly bad, mainly cuz the direction was to "emulate" those classic Adventure games and whatnot. I'm sure it was meant to be charming, but it did not work. On Mewtwo's end, he just sounds really bored. And the dialogue doesn't help either, kinda giving off the same vibe as Vegeta VS Shadow, only worse.
Mewtwo pretty much just stands there for most of it because his sprite lacks a lot of dynamic posing. The animation overall has neither character make any creative use of their arsenal, it's slow and boring, super short, and downright unfinished as you can clearly see a green screen background when Shadow powers up.
It all ends with a short, lackluster unseen fight between super forms and Mewtwo wipes Shadow's mind before killing him with kitchenware. It's cheap, it's corny, it's anti-climactic. Them coming to the conclusion Mewtwo wins is in and of itself fine, but even for season 3's research it's a baffling one, because they don't give Mewtwo any feats that suggest Shadow doesn't bum rush him and kick his head off frame one. By the stats given to both of them, it'd be the opposite result.
For the longest time, there was a misunderstanding that the animator for this one had actually made it bad on purpose, but in recent years he did speak to someone in the community about it and we know he was working in sub-optimal conditions and suffering burnout. It's not his fault, I really don't blame him, his health is the most important thing. But the end product was still incredibly underwhelming, and that's mainly what I want to rectify for such a pristine pairing.
"What would you wanna see from a redo?"
For one, actual combat. Mewtwo using his versatile array of moves and making smart plays while using mind games to his advantage, and Shadow not fighting like a moron, getting to use more esoteric Chaos powers, and some weaponry. Also, Mewtwo could use his other Mega evolution as well, and Shadow seems to be getting some dope extra Black Arm powers in October. Lot of cool things you could do choreography wise, I think it writes itself.
More importantly, I want their characters to actually come through. I will commend the original episode for that one part of Mewtwo protecting a Zubat, that was neat, but it stopped there. Beyond that, the characters didn't have, well, any interactions really, let alone meaningful ones that show who they are beyond walking natural disasters of grief and angst. Have baby Pokémon and Chao present, where both characters make active decisions to prioritize them over themselves. Have Mewtwo mindread Shadow and identify with his trauma somewhat. There's a lot you could do here, think Magneto VS Tetsuo and how well those two interacted and somewhat came to an understanding at the end there.
“What does the matchup mean to you?”
Both the Sonic series and the Pokémon franchise were prevalent in my childhood growing up. I always loved the colorful world of those hedgehogs as well as the various spinoff materials, and while I honestly don’t like playing Pokémon games that much, the anime had a chokehold on me for years.
Shadow in particular was always one of my favorite characters, and persists in being so out of the large roster they’ve used on the show. Mewtwo I always thought was cool as shit, even when he was piecing Ash up. I saw the matchup in 2016 and was hooked, it sounded like a ridiculous amount of fun. Then it wasn’t, so y’know.
Now to address some of the pushback I've seen against a redo, some valid, some not so valid.
"Shadow a FOURTH time?! Really?"
Fair. I can get not wanting to see this damn hedgehog once again when he's made his face quite known on the show. Tolerances for that will differ. For me, though, Shadow still has more to be explored, both in character and in abilities. Vegeta is a season 1 episode, and so doesn't really do a whole lot with either character. It's more of an abridged version of the characters really. As for Ryuko, it was a well made episode and easily the best, but it was held back by how limiting the matchup is on Shadow's end, as he couldn't really use a lot of his more esoteric abilities liberally as Ryuko's answers to them would just be...to try more cutting attacks. Also, Shadow is weirdly the most generic he’s been, not overly smarmy like in Vegeta or annoying like in Mewtwo, just kinda basic with the occasional edgy one-liner or reference. Okay stuff, but real surface-level. Ultimately still a thematic and fun pairing, but Shadow can and has done better in that department. Mewtwo VS Shadow might as well not be an episode for either of them. That's not Mewtwo and that's not Shadow. The way they fight, the way they talk, none of it works. So basically I just wanna see the matchup that uses Shadow to his fullest actually done well, and on Mewtwo's end, will also give him justice.
"I want Mewtwo and/or Shadow to fight different opponents."
Can't argue with this one. My personal favorite for both, but some would rather see things like Mewtwo VS BlackWarGreymon or Shadow VS Vincent Valentine. Totally fair.
"This is just so Shadow can win."
Nope! Frankly, out of any rematch they've done, this one would be the least geared to that kind of statement. There are a lot of moving parts with various scaling arguments, ability interactions, and specific details of their worlds, it's entirely feasible Mewtwo could just win again. Yeah, the episode is wrong by its own logic, but that's really besides the point. Of course I'd want Shadow to win, but much more than that I wanna see a hype ass episode for a hype ass matchup.
"We already have a Sonic episode in a few months, and a Pokémon episode next year."
Another understandable one. People get tired of the same world and the same franchise, variety is the spice of life and everything. Though, in all fairness, the incoming episodes for both are all-out total wars with both sides bringing teams of critters and robots to battle it out. They'd also hardly have the same vibe of this one. Of course if that's not enough justification for you, that's alright. I myself would prefer franchises get more spaced out, and am really just making this one exception. Probably wouldn't even be pushing this if not for the specific choice of a rematch.
"Well, what about [other potential rematch]?"
Eh, do what you want at the end of the day, I just know what I wanna see the most. Maybe I'll comment on some of the other commonly talked about options if people care to hear my take.
Alright, I think I yapped for long enough. This wasn't necessarily to make anyone viable for the vote suddenly gun for this one to happen, just an outlet for me to gush about it and also maybe justify why to some skeptics.
TLDR; Super dope matchup, it sucked, deserves to be good, both have more you can do with them, vote for what you want, Fulgore VS Sektor 2 supremacy, Bowser VS Eggman will be peak