Have the past Dragonball games been slowly incorporating all these new elements or did they finally just decide to go big?
Both, in a way. Like, what kind of elements? More than one-on-one has been a thing for a while; 2010's
Tag Vs. / Tenkaichi Tag Team from Spike on PSP (a portable version of the
Sparking! / Budokai Tenkaichi games) introduced 2-vs-2, and last year's
Battle of Z from Artdink introduced 4-vs-4.
2011's
Ultimate Blast / Ultimate Tenkaichi introduced a character creation mode and there was a slightly-original-new-story-mode to go along with your character, but it wasn't anything to this extent.
All of the MMO-lite elements are brand-new to the
Dragon Ball franchise here in this game (hub world, grinding for gear, etc.)... with the exception of
Dragon Ball Online, which leads to...
One question about the lore: why is Majin a race but we've also seen multiple official Majin characters of different race?
Not sure I quite understand the question, but:
The vast majority of the story here in this game is being adapted from the now-defunct Korean MMORPG,
Dragon Ball Online. This includes things like the Majin race (which... is kinda redundant, I guess). In a nutshell, Buu made himself a wife and now there's a bunch of them.
What other Majin do you think you've seen, beyond the various forms of Buu? If you're thinking of the villains under Babidi's control (Pui-Pui, Yakon, Spopovitch, Yamu, and even Dabra), they were more "under Babidi's Majin charm" rather than straight-up "Majin", so to speak.