The only way Davis sister could have deserved her screen time was if they made her a digi-destined as well, otherwise she was not needed. I feel her only need for existence was for Kari to yell at Davis for not appreciating his sibling more. And yeah Mimi was in the states, but even so she deserved more screen time than Davis' awful awful sister whose name I don't care for because she sucks.
The Dark Ocean and the Dark Seeds are probably the biggest plot holes in the series, as for the Dark Ocean from what I've read that episode was meant to be a special filler episode that was just supposed to be creepy and never resolved but UHP- they made it canon and ruined everything. Especially how Gatomon was able to Super evolve, HOW?! They explained they got rid of their crest power to seal he gates yet some how she's able to do it right off the bat! The crest thing was also just a weak explanation for why they couldn't super evolve, especially since the crests didn't exist nor were necessary since they were made to merely hone the Digidestined's powers! Hell, all that would do was depower them for a while, they prove themselves worthy of their element of harmony, and then get new super forms that get terrible toys based on them! While joking, it pissed the hell out of me that we never saw HeracleKabuterimon, that was my favorite trading card.
And the Dark Seeds is another can of worms altogether, that was actually a failed attempt to tie the show with the famed Wonderswan series. Prior to 02's events, Ken and famed protagonist and debatle Gary Stu Ryo fought and defeated Milleniumon: A fusion of Chimeramon and Machinedramon(believe it or not, less ugly than most Xros evolutions). It actually would've been cool if the whole series was building up to bringing back said villain. And you know who was one of his minions in the games? DAEMON! Which would've made his random invasion of Earth make complete sense...instead of making the viewers wonder why this happened and why wasn't he the main villain? It also would've worked seeing as how Daemon was the main villain in the manga the series was based on. Instead, we have Myotismon cloning Ken's seed(which just seems unnecesarry as there were like a million) which he shouldn't have known about given he was dead in during this time of events nor had control over as he wasn't the villain who freaking made them! And would he have the power to control them, he was killed TWICE before this and barely had enough power to control that creepy friend of Cody's dad. Final note on this, you'd think finding out what all the other spores did would be a sub-plot too but NOPE completely glossed over.
OOH- the destiny stone arc was just...just a waste of time. Not because of Blackwargreymon oh no, but because after they were all destroyed Mr.Dragongod REPLACED THEM ALL LIKE THAT! We wasted a whole arc on these things, and you could just replace them in an instant? Laaaaaaaaaame.
Gatomon made NO damn sense in 02, especially since her rookie form had almost zero screen time. She should've de-digivolved into Salmon, and stayed that way for the whole season and be self-consciense about not being a Champion.
Definitely yes to the evolution system, needs a complete over haul, especially with Omegamon.
As for Frontier I will admit it was probably the best of the even numbered series, but I feel knocking out the symbiotic tamer monster relationship and replacing it with Ronin Wariors was a bit of a stretch. Also Botamon and Neemon were just pointless tools for exposition and comic relief. And why didn't they have powers? Made no damn sense.
And on the topic of Xros wars, what bugged me most was not the evolutions themselves(though they were bad toy commercials, even for Bandai's standards), it was what the only thing stopping Mikey(not calling him his real name as its too obvious a tribute) was his own imagination. That...was kinda weak. Don't get me wrong imagination is great, but the other series protagonists had to go through numerous emotional and phsyical trials to master their evolutions and all he has to do is go around a loophole? Weak. And while I've seen just a little, it looked like he knew WAY too much about what to do with his Xrosloader, where's the room to grow?