Not sure how unpopular this is but I still believe Bandai was better served turning Tri. into a full season which could've served the purpose of appealing to longtime fans again while potentially drawing in a newer audience at the same time. To me there was enough material in the ideas/themes that Tri. brought up for 50 episodes if they'd delved into those fully and expanded upon other things.
The slice-of-life element that we saw early on and subsequent comedy that came of it was actually a nice touch, and I think it would've worked fine in Tri. had the pacing of the story been better throughout. In this regard I would've liked to see how the older Chosen Children tackled some of the challenges associated with maturing into early adulthood. Perhaps there was an opportunity to include a bit more character drama outside of simply fighting to save the world - how do they balance wanting more independence vs. obeying parental authority? Though not a romance-first series, how do they deal with teenage/puppy love (like Jou and his mysterious girlfriend for instance)? How do they reconcile with the fact that they're all slowly drifting apart as they get older and have their own hobbies/interests to pursue?
Had this series been longer there would've been more room to give the younger Chosen (namely Daisuke, Miyako, Takeru and Hikari) the character development they never really got in 02. A surprisingly large number of people were upset at their being sidelined the way they were in Tri. and to me there was room for them in any full-length Adventure sequel.
The reveal of two members of the very first generation of Chosen was one of my favourite parts about Tri. I like that they picked up on this little part of the backstory and made use of it, and wish they'd gone even further into this particular part of history involving them and the Four Holy Beasts.
A plot featuring Yggdrasil maybe could've worked but I always felt the way Tri. introduced it was so very random. It was interesting that they chose to bring Adventure closer to the null canon of the card games though. If we consider the Real World, how many other adults - other than Daigo/Maki and co., Oikawa and Iori's dad - were aware of the existence of the Digital World? Depending on their goals and intentions, maybe this would've sowed the seeds for a larger conflict between humans and Digimon.
Another smaller thing that Adventure alluded to was the existence of other worlds, which we saw in the Dark Ocean and that world of dreams. This has always been a fantasy series and to me dimensional travel would've opened up a lot of doors for other subplots. I would've loved to see them exploring more of the lore and fully introduce the idea of Witchelny into the story instead of leaving it as background material.
There was also potential to really mix it up and include a bit of horror in the series which could've been a rare first for Digimon - especially with Dagomon and his creepy minions - which saddens me as we never saw that expanded upon. There's a fair bit of Lovecraftian influence in the franchise already, so why not make use of it?
There was a lot of good ideas that Tri. brought up; I just didn't think they necessarily made the best use of them. IMO they went a little too safe with the sequel instead of taking a chance on trying new things.