Some consoles, usually for multiple reasons, underperform commercially and sell below expectations.

In this thread, we take a console that underperformed, and suggest changes to its design, marketing, software, etc to make it sell better than it did historically.

What is your pick, and how would you "fix" it?

Well, I have to fix 2 consoles to fix a company. So to fix Dreamcast I need to go back to the Genesis era. Never release the 32X. Saturn needs easy to develop for hardware. Better marketing. Proper Sonic game on launch. Good price point. Court Square to put FF7 on it. Holiday 1994 all regions.  Should help Saturn become a success. If we can fix that then SEGA has more money to deal with Dreamcast. So wait a year from 1998 to 1999 for a global release not just US. A bit more power in it and 32 MB of the main RAM on top of the 8MB VRAM. Faster CPU and stronger GPU chip. DVD player built in. 56k Modem. Dual analog controller. 9.9.99 $299. Get GTA III as an exclusive (it started development on DC) and buy Bungie. A month before PS2 releases promote to get $10 bucks off a copy of The Matrix on DVD. In Japan, this promotion lasts until a couple of months after PS2 launches. US it lasts from September to December 24th 2000. Dreamcast is fully BC with all Saturn games and controllers. (the Saturn in this scenario has entirely different hardware)

So many to pick. Dreamcast has been picked, and I don't want to pick Wii U because its failure did lead to Switch.
Let's go with PlayStation Vita if a handheld counts.
1. One SKU at launch. The 3G was unnecessary.
2. Support microSDs from any manufacturer provided they have a minimum write speed that's fast enough. 1 GB of storage included at launch instead of waiting for later models.
3. No rear touchpad. This would probably shave at least a few dollars off of the manufacturing cost of Vita. If the devs want it so desperately than release it as an add-on.
4. Connect to the TV using the port on top which went unused IRL.
5. No OLED screen, this will save at least a few dollars.
6. Secure IPs like Monster Hunter and Grand Theft Auto. These were sorely missing on Vita. Actually release BioShock as well instead of cancelling it.
7. Gran Turismo. How can you release a PlayStation platform and no GT? This would be a banger launch title, but within the first year at bare minimum.
8. When you launch the PSTV, give it feet so it doesn't slide around and include 4 GB or more of internal storage. When the Slim launches, that also will have at least 4 GB of internal storage.
9. Open up the floodgates for more PS2 ports. It would've been nearly impossible to emulate PS2 on Vita, but first and third-party ports could've sold like hotcakes. There's a big appeal for playing PS2 games wherever you want with a higher resolution.
10. L2 and R2. Ideally they are triggers, but even having them as buttons is okay.
Even if all these changes help, I don't see Vita selling much more than Sega Mega Drive or Nintendo 64. The ceiling of Vita seems around 35 million given the success of 3DS (Pokemon & Mario) and the growing reliance on smartphones and tablets for small games.

I think the PS3 could have taken a while longer in the oven for better and cheaper components (say, 65 nm Tesla uarch) and a more compelling line-up.

My plan for Wii U.

On the side of the hardware, I'll just make it slightly more powerful, and change the design a bit so it doesn't look so much like a Wii that people confuse it with it's predecessor anymore. The CPU is 15 years old by the release of the Wii U and really not up to the task anymore, a 970 derivative, which is also 64bit CPU and "only" 8 years old by then, would have been much better. Also the GPU is slightly too weak, something more akin to an underclocked Radeon HD 4750, so basically a Radeon Mobility 4830 would do while keeping the power draw about the same). With this improvement, the Wii U would be able to connect more than one Gamepad to the console, something that would have been very helpful for the console and some games, and would be able to run PS360 games without slowdowns due to it's lagging CPU and instead can run their games in beautiful, as in, not choked down by tiny 256/512MB RAM.

Now that the hardware is fixed, let's move on to the main culprit of it's abysmal failure: The name, the design and especially the marketing. Actually call it a new console, make it somewhat more distinct in it's design so people actually can see it's not just a Wii on the box but a new console, and the tablet is not just an add-on for the Wii but actually part of a new console generation.

Immediate one that comes to mind is the Xbox One. Console was “dead” before it even launched. Keep the Kinect 2.0 as an optional accessory but get rid of the Kinect requirement to bring the price down to $399 at launch, the entire showcase presentation of the console be focused exclusively on games rather than it being entirely about being a multimedia device, drop the online DRM that limited game sharing/resale, give it a proper console associated name rather than “One”

I also wouldn’t allow for Sony to get CoD marketing that gen, keep a closer on eye on 343 and don’t let the disaster that was Halo MCC happen along with Halo 5 pretty much ruining your biggest IP. And after Insomniac finished Sunset Overdrive, accept the Marvel Spider-Man deal and partner with Insomniac again to make it exclusively.

Would it have saved the Xbox One from being demolished? Probably not as there would still be a major first party issue throughout most of the gen outside of that first year, but that would’ve gotten at least an extra 10-15 million units in console marketshare, if not more just by having a completely positive pre-launch announcement.

I would add launching it a year earlier with Halo 4 as a cross gen launch title. So it would have a year headstart over the PS4 like the Xbox 360 did with the PS3. 

I'll go chronologically.

1. SNES - Support it with software and marketing for about two to four more years. Let it die naturally like the NES, don't force it. Could have hit 80 million.
2. Saturn - Cut bells, whistles, and price.
3. N64 - Use CDs, not cartridges.
4. Dreamcast - Anti-piracy measures - no platform was hurt as much as this one by piracy. Using Sonic Adventure 1, sales on other consoles, and year 1 Dreamcast games as a reference point, Sonic Adventure 2 sold about 10-20% what it could have without piracy if the console didn't sell any more than it did.
5. Gamecube - Make a Nintendo console instead of a budget Playstation for kids.
6. Wii - see the SNES solution + don't kill services.
7. PS3 - use a cheaper chipset, cut costs. That's what made PSX and PS2 great.
8. 3DS - Lower price at launch significantly. Perhaps launch a year later after the anti-3D hysteria subsided. Add that 3D TV channel many expected. Launch with a killer app (Animal Crossing could have been that).
9. Wii U - should have been Wii HD and an extension of the Wii platform. Should have launched with a killer app, Nintendo is usually good at having their finger on the pulse for launch titles/relaunch titles that will be killer apps (SMB, Wii Sports, Breath of the Wild, etc...)
10. Vita - Needed a killer app, or at least some system sellers - Sony tends to be great at this when it comes to features, but struck out here.
11. Vita/3DS - I think both suffered because of the sudden boom of mobile gaming which drew most heads for a few years. The iOS App Store took off just before the launch of these two systems, Switch came out when heads realized there was still a place for a dedicated handheld gaming console in their pocket/laptop bag. Not much to be done about that, just bad luck.

Do not try to fix WiiU. That was a clear disaster since its presentation to the public.
Not only because used a CPU and GPU extremely underpowered and totally amortized by then, or because its chaotic reuse of the trademark and logo of the Wii with a confusing "u".

That machine didn't even get basic concepts of gaming, like treating all the players equally: the controllers in WiiU were a conceptual mess : The machine could manage only 1 WiiU controller, you cannot buy another one, and the other players had to play with the controllers... of an older console... creating a market chaos for parents and greatparents.
And trying to make players play with 2 screens meters away, at the same time, was also not a good idea,

Sincerily... some machines undeservedly fail, like the Gamecube (well, Gamecube failed cause some things Nintendo did wrong, but the internal hardware was great).
But WiiU was clearly a disaster since the first minute of its presentaion in that E3: all was a bad fever dream with no sense at all. And many saw it,,, and sayed it. Including me.

You cannot fix the WIiU, because WiiU was a conceptual clusterfuck mess, only to be good as a gimmick thanks to some lazy mini-games collection,,, for 5 minuts max.
Or as an emulator machine for classic games, using the screen on the pad.

I'll go with the Wii U.
I would...
Change the name to the Wii 2.
-Have a better CPU. That CPU was weaker than the Xbox 360's. The GPU was fine, so the system was CPU bottlenecked. RAM was fine too.
-Get more 3rd party engines like Unreal Engine to work with the system.
-Get as many 3rd party games on the system as possible and make sure they're better than the PS3/Xbox 360 versions. Show off that gamers need to get this system for better 3rd party games and have great Gamepad gimmicks to make the games better. There was no reason the Wii U only got Mass Effect 3 when the other two got the whole trilogy around the same time at the same price, and missing some dlc. That goes for a lot of other games where content was missing.
-3DS integration. What I mean by that is more games that are on both systems needed cross play and cross save. Along with cross-buy. Make people who buy Nintendo handhelds (which always do better than the home consoles) a reason to buy your home console.
-Transferring Wii software to Wii U. I see no reason why you need to go to the Wii menu to play Wii and WiiWare games. Everything should have been under one menu.
-Speaking of VC and WiiWare games. Those games should have all been on the Wii U Eshop similar to how the DSiWare games were on the 3DS Eshop. The Virtual Console makes no sense to me since it's all emulated and ready to go. No reason to start the catalog from scratch. Those games also should be cross-buy.
-Better marketing. Actually showing off the console so people know this is a new console, not an add-on.
-Better 1st party launch games spread out. Nintendo's games weren't ready so there was a big drought between launch and summer of 2013. Lack of 3rd party games didn't help, but as stated above they needed to go harder.
-Getting Pokemon games on the system. Pokken Tournament came way late and most people wouldn't know that's a Pokemon fighting game. They needed something either like Colosseum or Battle Revolution. Games that can connect with the main series games on 3DS.