I know there's two Digimon MMO's out there already, but bear with me on this. Also, I've posted this damn near everywhere else already, so I'm gonna copy/paste it here. But I will include an image to boot.
You know, I think there needs to be a more permanent, established, and well-rounded Digimon MMO.
Not based on a specific season.
But more around Digimon World.
But not any specific Digimon World game.
Taking the best aspects of each and making a nice compromise between them all.
For instance, you can have, let's say up to three Digimon at a time. In addition, they all level up with various stats and special attacks as they battle.
However, you have to actively care for them with food and, yes, toilets, as you raise them, and the stats you train them in or focus on when you give them equipment and when you level them up can affect how they evolve. You could end up with three of the same Digimon, or three distincly different Digimon.
And, just to keep the "one on one" nature of the series on a whole, you can only send a single Digimon into battle at a time, but you can switch between them.
What would you think about something like that? Would you play it? I know I would.

And, something I may have forgotten, there's also the question of boss fights. I understand that most of the "boss Digimon" would be possible to actually have as one of your partners, so the "bosses" would essentially be amped-up versions of those Digimon, with different attack patterns and overall higher stats. In addition, there's also the nature of the anime, in which the biggest fights are typically every protagonist vs. a single enemy. And that's where forming parties with other players would make a lot of sense. Since you could only actively battle with one of your Digimon partners at a time, it would encourage you to form parties with other players. Basically, having the kinds of roles you'd expect in a standard MMORPG but with Digmon. You would form strategies, work with others, and take down the huge bosses together. And with a set world just for the game, they could expand on it a lot more. There could be story parts where you fight specific groups of Digimon, like the Devas or the Royal Knights, or the Four Great Dragons, or even the protagonists of either a game or one of the seasons of the anime if it gets far enough. I mean, the characters have all come together in a world before, so who's to say they can't do so in this kind of situation? Personally, I think it would be pretty awesome. Even if it is my idea.