Smash 4 is considerably limited in order to have it's 8 player smash and 60fps.  Anyone can see that when you look at the character models when they do their victory pose.  Smash 4 could have looked considerably bettern if the Wii U had been more powerful.

Mario Kart 8 makes numerous concessions when doing 4 player split screen and yet still can't do 60fps. 

Super Mario 3D World has frame rate snags when a lot of stuff happens and that's not even that impressive of a game technically!

Pokken Tournament's backgrounds and resolution speak for themselves I think.  Definitely very restricted.

Wind Waker HD and Twilight Princess HD have numerous framerate snags when particular effects hit. 

Also, I don't think I should have to list out every single solitary game that has obvious undesired limitations to it due to hardware to show that Nintendo reall could use bettter hardware for their graphical style.  We've seen games with graphical styles similar to Nintendo's that show just how far you can take that look.  Just look at the new Ratchet and Clank which is just crazy detailed and yet still in that similar style.  Nintendo can use art style to create something that looks great, don't get me wrong, but the fact is Nintendo underestimated what they needed in terms of hardware muscle and just how fast they would hit the limits of the machine with their more ambitious titles.  With a PS4 level machine the games they currently have could look better with greater detail drawn at greater distances and sharper framerate and better AA. 

And Xenoblade and Zelda are NINTENOD IP's.  Their ability to realize their ambitions is not some minor quibble, it's a very important consideration when discussing hardware.  If their machine causes considerable trouble for the developers of those IPs due to hardware limitations than that's not a minor gripe, that's a major miscalculation.

Nintendoland and Pikmin 3 did look better than NSMBU, but they still kind of felt like tentative first steps into the HD realm compared to later games like Mario Kart 8 and Captain Toad Treasure Tracker, which look a lot more sophisticated.

Just because their are frame rate drops or textures aren't the best doesn't mean that it's the best the Wii U was able to handle. A lot of the games are simply limited by the time they have to develop it, and the size of the team that is put on the project as well as game engines and how well they work with the system. Star Fox Zero has frame rate drops and low textures, yet you would be hard pressed to get me to believe that's the best the system could handle. There is no coincedence that games look better as the gen goes on, and this is literally always the case. 

I mean, you hardly ever see any real sizable updates, and almost no game used the ability to install before playing with the exception of Xenoblade. And these are pretty much the standard in how games are handled this gen and have been handled last gen, and guess why.

And that install only erased a handful of problems and helped with pop in.  Didn't improve the crude AA, add anisotropic filtering, fix the absolutely awful problems with pop in in the city, enhance the polygon counts of various models both architectural and fauna related, improve the textures of the general landscape and city. etc.  I'm not knocking the game, I think it looks great aesthetically and is very well accomplished for the Wii U, but that's the problem, it runs up against hardware limits pretty hard.  Also, I would say Mario Kart 8 puts forth a very impressive presentation on an asset-to-asset basis, but the concessions made are abundantly clear when you look to background/surrounding details which vary from basically functional to extremely basic. 

And my point wasn't "Nintendo's games look bad," but rather that Nintendo clearly has the development chops and ambition to have far more detail in their games than they currently do, and that they have several games that are clearly running into Wii U hardware limitations that they obviously didn't anticipate giving them such troubles. 

I don't necessarily disagree with the overall thrust of your argument, but a few things spring to mind.

Regarding Smash, I think running at native 1080p is the bigger drain on Wii U's resources than having 8 character models on screen. Wii U is primarily a 720p machine, so doubling the pixel count comes at a cost. Overall though, it doesn't feel to me like a game that was significantly limited by Wii U.

And not many games that I can think of, on any platform, run at 60fps in 4-player splitscreen

As for 3D World, while it may not be the most technically advanced game on the system, (after all, it came out only a year into the console's life) it runs at a practically hard-locked 60fps:

In my view, while 3D World was Nintendo's first Wii U game where it felt like they had a confident handle on HD development, I think it was Mario Kart 8 that was their first game where they started to hit system limitations.

Oh I know those three were petty, but I was trying to make a point that even at the most efficient levels of using the Wii U's potential, you can still see the game running into limitations. 

Smash I will stand by though.  It's very easy to see how numerous stages and all the character models could have looked better, much less lighting and effects.  It's not ugly by any standard but could look sooo much better. 

You could say the same for PS4 games though, for example, that Uncharted 4 could have hit it's original 1080/60fps target if they had stronger hardware. All hardware has limitations. 

There are definitely Wii U games where it feels like the hardware is strained, (XCX and Breath of the Wild come to mind) but for the most part I feel like Nintendo's rather modest ambitions were well within the console's capabilities.

Yes, I remember they said with MK8 they pushed WiiU hardware to limits, of course they most likely surpassed those limits with X and Zelda BotW, but those game are much dfirent than MK8.

Yeah, MK8 seems to be GPU bound, as it sacrifices AA, while XCX is more memory-bound. 

Breath of the Wild will likely end up being the game that pushes the system the hardest. I mean, FAST Racing Neo probably edges it out in terms of GPU utilization, but BotW is much more CPU and RAM intensive with its open world and physics systems.

Yeah, but sacrifices had to be made to make the game as big as they did, hence why some games have anti aliasing (LoZ Breath of the Wild for example) and some don't. It's pretty much my point. There is really no game we can go to and say "this is what Metroid on the Wii U would look like" and again, it is a sad thing.

The real fact of the matter is their is ALWAYS going to be hardware limitations. Even if the Wii U was as powerful as the PS4, there are going to be limitations, and it's nice to see how franchises take advantage of the hardware that is available vs what the franchise looks like in its next generation. The small niggling issues in Xenoblade Chronicles did not hamper my enjoyment of the game in the least. In fact I would go as far as to say I thoroughly enjoyed it dispite its shortcomings and am glad I got to play it now (and again prefer it) instead of having to wait until whenever it came out when the NX does arrive. That in turn increases my interest for the next installment when it does come out on NX, to see how that one stacks up.

Ultimately, what limitations would Metroid have on Wii U? Other than it would be 720p we'll never know for sure. How good would it have looked? We also will never know. Same as asking what limitations will it have on the NX (if it ever comes out)? We don't know. So you're asking for a solution for a problem you have no clue the game has (and other franchises like Fire Emblem, Kid Icarus, whatever IP we will never have the luxury of seeing on Wii U).