My advice would be to stow them (or one of them) somewhere safe, in a sealed bag, where it won't get exposed to sunlight or dust or moisture, and stays in mint condition.

Many years from now, you could get a crazy price for it. 

In America maybe: here in Europe can be found with ease. Which sucks for me, because I'll not be able to sell it for a ridiculously high value xDDD

Which ones?

I've seen almost all of Wii U's retail releases in stores but I haven't seen a trace of Devil's Third anywhere.

New Super Luigi U is quite expensive for a Wii U game, at least in Sweden. I've seen them go for as high as 100$ on the used market. That was a bit extremet though, 70$ or so seems to be what it goes for. Pikmin 3 before the reprint was also expensive.

It depends if it eventually stabilises at those prices. In most cases, 'rare' titles are listed at enormous prices for the days/weeks following release before they eventually stabilise at a more comfortable price. I haven't been following the US version of Devil's Third since a PAL version exists, but I'd expect it to have the same trend as most other rare titles like Breach & Clear and Retro City Rampage DX (US$200+ initial prices, before settling around US$60-120).

It looks like NOE and NAL overestimated demand though. It's been available at pretty much every Australian, New Zealand and UK retailer since it launched back in August.

Since we don't know accurate production/shipment figures for any titles, we don't really know. However, if the NTSC-U/C shipment is around the 1-2k mark, it's likely to be one of the rarest NTSC-U/C Wii U games. Only other one that comes to mind would be Giana Sisters: Twisted Dreams - Director's Cut, but that's still available on Amazon at the moment. After that you have some less common titles like Deus Ex: Human Revolution - Director's Cut and The Voice: I Want You that some people have described as rare, but both are still readily available at various retailers.

It's more interesting in the PAL regions and Japan. Rarer titles in the PAL region include the country specific titles (i.e. those that didn't get a wide release throughout the region) like Baila Latino (Spain), Cocoto Magic Circus 2 (Germany), Fit Music for Wii U (Italy), Hunter's Trophy 2: Europa (Germany), Luv Me Buddies: Wonderland (Italy) and Scribblenauts Unmasked (Australia). Then there's some third-party multiplats like Pac-Man and the Ghostly Adventures, Sniper Elite V2 and Zumba Fitness: World Party that are becoming harder to find. After that you have titles like Jett Tailfin, Project Zero: Maiden of Black Water, Rodea the Sky Soldier and Teslagrad that could become rare in the future; Project Zero was only released as an LE in Europe, but a standalone release with a barcode instead of "NOT FOR INDIVIDUAL SALE" was released in Australia.

I was thinking of the physical version of Fatal Frame V in PAL regions, and also, some odd indie games that got a retail release for some reason (Jett Tailfin, or however it is called, it has a retail release in Europe, no idea what is it about, though).

Yeah maybe those could become super rare and valuable for collectors, especially Fatal Frame. I actually think Wii U will have a really expensive library in 10 years, I even made a thread about it sometime ago.

yea wtf is going on? http://www.amazon.com/Devils-Third-Wii-U/dp/B00KWIYS3K/ref=sr_1_1?s=videogames&ie=UTF8&qid=1450577759&sr=1-1&keywords=devil%27s+third

Earlier this year I bought the Legend of Kay Anniversary Edition for Wii U. That's got to be pretty rare.

NA version will