So, obviously the core appeal here is the Pokemon Fusions. This idea is kind of self-evidently really cool, and there's a lot of room for experimentation, both with mechanically interesting or busted combinations, and seeking out combinations that come with really cool sprites. Unfortunately, I find that these two goals clash pretty hard. Pokemon has always had a tension between optimisation and aesthetics, but it comes into sharp focus here, especially due to the fangame difficulty (more on that later). I find it frustrating how often I went to check a fusion, and only the vastly worse version had a custom sprite. I understand that the custom sprites come from community contributions, and the number they have already is beyond impressive. It's just a shame, kind of a natural limitation of the concept, that you're never going to have even a tiny fraction of the fusions look good. On the topic of natural limitations, this one is just bad luck on my part, but so many of my favourite Pokemon aren't in the dex. I totally understand the requirement for a limited scope, every single Pokemon added is exponentially more data to store, it's just the choice of which Pokemon get in to that limited roster that doesn't resonate with me personally.
Overall, I've always found the play experience of Pokemon Essentials games to be pretty janky, and this is no exception. Long loads, abrupt transitions, non-looping music, busted collision everywhere, and it's almost impossible to move exactly one tile during speed-up. But, again, I understand the limitations here. No GBA ROM can ever contain the data for [420 choose 2] Pokemon, even with all the decompilation efforts that make ROMhacks otherwise a much better choice these days.
As someone with a lot of backlash and bitterness for genwunning, it's a great disappointment that the game is set in a retread of Kanto. I'm grateful Modern Mode exists to at least update the Pokemon distribution, but it can only do so much. I guess I don't know what I would have preferred - an OC region would vastly distract from the game's focus, and any other canon region would feel like a particularly arbitrary choice. Still, there are some inexplicable archaic decisions left over from the originals, like Lt Surge's gym puzzle or the presence of Sokoban puzzles with random encounters in Victory Road - especially baffling since other things are changed, like Saffron being gated by Rocket events in Celadon instead of. there only being one source of potable water in the region. I would have liked to see more of the experience modernised and streamlined, especially since the vast number of possible Fusion combinations should make the game extremely replayable otherwise.
As stated, I played on Modern Mode, so I can't comment on the vanilla, hopefully better-designed gameplay, but the important Trainer battles in this game can be kinda wack sometimes. Modern Mode changes every Gym's type, which makes sense for replayability, but dialogue is unchanged so you have to just sort of intuit it from the Gym trainers. And then for the Elite Four, you're going in without any idea. The Elite Four and Champion are also way overtuned, with Legendaries a plenty and a sharp level spike. Peak Pokemon fangame design. The Giovanni fight in Celadon is also a sudden level bump. This game shouldn't try to be particularly more difficult than canon, finding overpowered fusions should enjoyably break the game, not be an requirement to succeed without tedium.
I find the OC plot insertions to be lightly cringy but mostly inoffensive - and as with the Celadon example above, sometimes positive. Still, it ties Fusion into the game's plot and keeps things mostly coherent, and it otherwise probably the game's least important focus, so I'm not fussed.
Overall, the concept is incredibly novel and executed... well enough to enjoy, but there are enough marks against the game in various areas to keep me from being terribly enthusiastic about it as an overall experience.
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I haven't played much of the postgame yet, but I had to come back to amend my review because it is absolutely awful so far, all of the worst parts of the main game distilled. Modern Mode continues the legendary spam more vigorously than ever, with random trainers on the road now having them. There are multiple Zekroms just in the one Gym! And I'm supposed to take a subplot about Eusine searching for the one Suicune seriously when the Gym Leader just has a second one? A subplot in which, might I add, Eusine's characterisation is completely inconsistent both to the canon games and just between different cutscenes.
The dungeon design has also taken a nosedive into tedium. I hate both Gym puzzles I've seen so far. One is a trial-and-error Simon says game, and the other involves wind periodically pushing you off platforms - with no visual cue, a bad audio cue, and the wind firing far too often, with a fall being a punishing waste of time.