Some thoughts on the lost memories of the Digimon. For one, I'm VERY glad they didn't get them back yet (And I kind of hope they stay gone, but I somewhat doubt it...)
Even if the 'bond' between partners was repaired easily, the actual -memories- are still gone. For something like a Digimon, who gets reborn when they die constantly... That's as close to death as this world allows. The kids' partners all essentially -died- and got replaced with reasonable facsimiles. That's pretty gutsy for this show.
Now, you can argue that the kids aren't taking it that way and since the facsimiles are all basically unchanged in persona, there's no actual 'Loss'. But I think that's why they spent 3 'episodes' dragging out Piyomon's acceptance of Sora. The cost for losing in Kokuhaku had to be paid. And Sora was the one who would be affected by the loss of the -memories- rather than merely the -presence- of her partner the most. Takeru certainly has a complex about losing his partner, but the bond they share is more innocent and emotional rather than based on any kind of shared history. Same for most of the kids (I think only Yamato has an argument for having a more 'shared experience' based relationship with Gabumon, due to how much he actually confided in him). Sora's sole character trait is how much she doesn't share her pain with anyone except Piyomon, so all those moments built together mean a whole lot more than just the unconditional support most of the Digimon partners offer just by -existing-. I think it was the right choice. I think if they spent the whole movie showing us the cost of losing their memories with all 8 kids (7 really, Hikari gets a net positive here), they would have had a very dour and uninteresting Digimon story indeed. And I appreciated the bits of humor, they were some of the best parts this time around.
I feel like there's this stigma in academia and in the reviewing world that 'crossover' stories are inherently awful and should never be taken seriously. I hate that stigma. And I feel like it's so prevalent that there's no one that even attempts to make GOOD dimension-hopping stories. That is such a shame, there is no story element that is ALWAYS inherently bad, and I feel like 'crossover' stories shouldn't have to die on the pyre of public opinion where there's so much potential there.
Yggdrasil represents this crossover element in a big way in the Digimon fandom, almost as much as Ryo/Milleniumon did way back before Savers. Now that Yggdrasil is the face of the idea of the multiverse in the franchise, there's so much backlash simply at what Yggdrasil represents. I know some of it is earned due to some questionable appearances (and some unquestionably bad ones), but I don't think the way tri is handling it so far is poorly done.
I theorize, along with many in this thread, that Yggdrasil (and probably Meicoomon and the Royal Knights) are from another dimension in the Digimon Multiverse. The earliest movies somewhat hint at this, and Meicoomon's innate ability to open distortions, as well as Yggdrasil's previous appearances hinting at his multiversality, seem to suggest that's a strong possibility. Some people seem to hate this idea out of hand. I'm personally fascinated by what they may do with it. We don't often see multiversal threats in the Digimon franchise despite it being a fairly consistent undercurrent in the games and manga. This would be only the second time the anime deals with it directly. Now while the first time (in Hunters' finale) was pretty much the epitome of why people say crossover stories don't work, the uses of it in the games and manga were much more deftly handled. Cyber Sleuth, Re: Digitize Decode, and the Xros Wars manga are often cited as among the best the Digimon Franchise has to offer and each of them featured plots that involved the multiverse/crossover elements.
I think Tri just might be up to the task. As others have also pointed out, they've already positioned Yggdrasil in a new light from its standard role of "tyrannical enslavement is for the betterment of peace". Instead, it exists in opposition to the Homeostasis. And with the crazed Gennai as its sole speaking avatar, there's a number of directions Yggdrasil could take as a main villain... (Or even secret ally? I still think Homeostasis's slavish devotion to balance is far more menacing in its own way...)
Re: Original Chosen Children, Maki, and Daigo
I don't see how anyone can watch Sora going through three episodes of Piyomon's unintended abuse but still come out stronger for it; and then see Maki go through 1 scene of it with Bakumon to only basically crack; and not realize that the entire purpose of the Original Chosen Children plot point being revived was EXACTLY for the purpose of showing the parallels between the kids' growth and the possibilities the adults represent.
This isn't just some fanfic wank to devalue everyone's pet theories about a two-minute conversation from one episode towards the end of season 1. There is actual narrative purpose in developing the characters of Daigo and Maki, and they deftly managed to tie it all into that infamous bit of lore that people have been obsessing over for literally two decades.
That's not fanfic, that's writing skill I genuinely applaud. (...or at the very least, it's GOOD fanfic, depending on how you use the term.)