Every video game would be fun and Western games would no longer be shit.
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More serious answer: It would be too much for the Big N to control all at once and not a whole lot would change. There would be only one video game console, one handheld, and games would either stop coming out on the PC or that'd be the alternative to those two. I'm not sure. Some video games that otherwise wouldn't be localized would, and some video games would never ever get localized. A bunch of porn games would stop getting made, no one Nintendo 1st party series would ever die (F-Zero would still be here for instance), and there'd probably be more actually BAD main series Nintendo games made, since Nintendo would probably allow some companies to mess with its games. But that's okay, because more main series games would likely get made.
but with their games they just stick to milking the dusty teats of their tired mascots with the same stale game play.
Every 3D Mario game is extremely different from each other.
Nearly every Star Fox game is very different from each other (and people hate them for that unfortunately).
As for other series, even if the games mostly are the same, they were good to begin with so minor changes will only make already good games better or slightly worse. It pretty much guarantees that a game in a series will be good. The slight changes are interesting anyways. Pokemon finally going 3D, Zelda going open world....
EDIT:
Also, Nintendo does do some different stuff on occasion but usually it just fades into the background. Think Glory of Heracles for instance. And they are getting better about that anyways, with Project STEAM and Splatoon coming.
Also Nintendo hasn't released a Metroid game in 4 years, and it was Other M, which is significantly different from previous Metroid entries.