As far as I know, Nintendo already develops and has always developed its own operating system for their hardware. The only company I can think of that used an OS not internally developed is Sega with Dreamcast which used an optimized version of Windows CE and apparently, that wasn't even its main operating system.
It sounds like you're just describing what you want to happen rather than basing this on anything in particular. The entire Mac line, besides the white Macbook, is carved out of a solid piece of aluminum. It's sturdier than plastic, but it only really makes sense for the Macbook Pro line because laptops are meant to be carried around so that sturdiness actually serves a purpose. For the iMac, it's just for show. That said, it'd make more sense for the shell of the 3DS to be aluminum, but seeing as it's not only plastic but Nintendo cheaped out again and made the screens, particularly the bottom touchscreen, out of plastic (rather than more scratch resistant glass), I presume Wii 2 will also employ a plastic shell. There's just no real reason for Nintendo to use an aluminum shell for their next console and "it looks cool" is not a good enough reason for typically cost-cutting Nintendo. The only thing Nintendo really copied from Apple was the minimalistic glossy white design for Wii and DS Lite so that imitation is hardly what I'd consider a "habit." Everything else Nintendo does is decidedly anti-Apple. If you just mean colors, then yeah, I think Nintendo is past their whole we-need-to-stand-out-so-let's-make-our-products-purple stage. A black matte finish shell would be my guess as it's simple and likely the cheapest choice.
I actually don't expect Wii 2 to be the same dimensions as the 360 slim. That still seems too large, especially if Nintendo moved away from disc-based media.