There are a few things that aren’t connected to any of the series, or rather are connected to all of them. I won’t care about spoilers, so you be careful.
I remember only fragments of the first season, Adventure… It was ages ago, watching it after school in german on RTL2. The kids were so many. Nine-ten? And they were so kids.
I guess it must’ve been hard for the writers to create a storyline where they could fit in all the characters, develop all of them well and still stay entertaining. Probably that’s why the two youngsters of Adventure come back in 02. In 02, the eight kids (counted, eight) of Adventure were made helping background appearances except for the two of them who became members of the new five person team. They were later joined by one more kid, a past enemy, going to six, the same pattern as in Frontier. In Tamers, in a totally different world, where Digimon is “just” a card game, the new team goes down to three kids, and they spend much more time in the real world, compared to the first two seasons or Frontier. In that aspect, Tamers and Savers are very similar. This number three seems to be the best one, because in Xross Wars the team is again of three kids.
The main character is always courageous, outgoing boy, wearing goggles (except Savers), having the element of fire (i’m not so certain about 02 and Xross Wars) and a small dragon type digimon partner (naturally except Frontier). Then there is a second guy, who is either “blue type”, with a light or ice element and a wolf digimon (Adventure, Frontier, Savers) and a smart and cool attitude, or a “green type”, with a more gentle personality and “elderly” wisdom, who is not as much of an opponent to the “red type” leader boy as the “blue type” boys (02, Tamers). Then there is certainly a girl, on whom at least one of the guys have a crush. So far it seems that Xros Wars will concentrate even more on the “red type” boy, degrading the others to support characters.
The digivolution is changing over the series as well. Adventure is the Beginning, to what all other are compared. I guess 02 had to bring in something new so that the good old scheme gets a bit of fresh blood, introducing armor and DNA digivolution. Armor digivolution is a great idea, and in later series, the modifier cards in Tamers and hybrid evolution in Frontier is very similar. DNA digivolution brings in the idea of merging, which is later continued in the Tamers mega digivolution and in Frontier. After the digimon-less Digimon of Frontier, Savers returned to the roots, first having an Agumon as main character’s partner and stripping all the additional digivolutions. It’s just the plain old thing from Adventure, where the strong emotions of the kids help the digimon evolve. The difference is, here they call the strong emotions and the crests of Adventure DigiSoul. Xros Wars is similar to Tamers in that digimon are “mixed” with something (in Tamers, upgrades from the cards, in Xros Wars, other digimon) and to 02 in that the digimon merge.
As for the storylines and character designs, the only exception is probably Savers. It’s clearly intended for an older audience, unlike the clearly childish environment and message of the other series. Probably that’s why i liked it that much.