Honestly, I think most of 02 needs improving, it is a fantastic example of how not to construct a sequel! If I could make just one change it would be to have far more involvement of the other characters, especially the older children either in affecting the plot or alternatively in keeping to 02, make them somewhat useless at the beginning and build their character arcs from the consequences of that. I just found that above all else very unsatisfactory even as a kid. Also some of the best character moments in Digimon were when the family of the kids got involved such as the Myotismon ark of the first season or most of Tamers. Aside from Cody's family or Jun who acted as the comic relief, their families were hardly involved at all which I think was a lost opportunity.
If I could play around with the season a bit more....
In the Emperor arc, for the first 16 episodes or so most follow the same format of 1. the children get stuck in a situation. 2. They find a Digiegg/mental. 3. The day is saved and they head back to the real world. I know its a kids show but like the Etemon arc in season 1 or the Arukenimon and Mummymon arc after this, it becomes repetitive very quickly! One way to change it up could be to have most of the older kids Digimon captured by Ken from the beginning, the kids losing to Ken at points with consequences and not being able to as easily go from the human world to the digital.
For the second arc I would probably keep Arukenimon and Mummymon as they were but perhaps have less screen time. Instead involve and develop the "Dark Ocean" far more then what we got from the series and have Daemon be more Myotismon-esque villain. But primarily the main focus would be not to have the "end boss" of the final arc come so out of left field like in both adventure seasons.
For the third arc I think Oikawa is compelling as a character as who wouldn't want to go to the Digital World. You could have the final clash between those whom the Digital World has chosen against those who believe that everyone should have the right to go. You could also have some of the other siblings of the Digidestined on Oikawa's side as who wouldn't be envious of their sibling if they had a Digimon partner and you didn't. I would however get rid of Myotismon as the end boss and perhaps use him elsewhere.
As for characters, keep the cast but give all of them an actual character arc! The only one who had any meaningful change to their character was Ken and speaking of, I would change his character slightly from a "I am EVIL...MAHAHAHA!" whom we only later discover thinks the Digital World is a game to have that be far more emphasized from the get go. There have been many occasions where if we acted out what we do in video games we would (rightfully) be called evil/psychopathic etc.