Systems that released too early or too late

[Wman1996]

These are my initial thoughts. I might pick some others later.
Dreamcast: Delay the launch in all regions by at least a year. Make the specs a little closer to what the PS2 and the others did, and use DVDs. The Dreamcast probably still would've failed, but perhaps not as much.
Xbox 360. Xbox was only 4 years old and faulty manufacturing led to more hardware failure than is acceptable. It also had no HDMI port, which would've been smart futureproofing since most 360s have it. November 2006 would've been fine, especially in the likely event the PS3 still made a bunch of dumb mistakes.
3DS. Delaying it until early 2012 might not have been necessary, but November 2011 would've been great. The 3DS was priced too high, had a sparse early lineup, and no eShop at launch.
Wii U. I'm well aware that the Wii was experiencing droughts in its last years before replacement, namely in 2012. But the Wii U had a very sparse first half of 2013, even for Nintendo games. It would've been better to cling onto the Wii for another year longer and release a few lower to mid-budget Nintendo games on it. Nintendo could then launch the Wii U in November 2013 with a very solid line-up, $300 price tag (no $300 vs $350 options), and an easier to develop for architecture.
PS5: Global supply chain crisis which still would've impacted a November 2021 launch, but would've helped a little.
Xbox Series X/S: See PS5.