I'm sure the Wii U will support dual layer discs, and even if it doesn't at first all it would take would be a firmware upgrade to make it happen somewhere down the road. But then again, this is Nintendo we're talking about here. It also would have been a trivial thing to allow DVD playback on the Wii, and that never happened... so who knows?
As for the xbox having only onboard flash, I keep saying that would be the low end "arcade" SKU. Its not the default model. Its just some cheap gimped version which any serious gamer would have to buy a HDD add on for. The point of it is just to sucker people into thinking they are saving money, but then after they get it they realize they need to spend another $100+ on the proprietary HDD add on, so in the end Microsoft gets the same amount of money out of people, but just in installments instead of all at once.
Why is there any reason to doubt it? Microsoft is already doing this right now with the 4gb Arcade 360. Its not like I'm speculating on something which is completely unprecedented. They've already done it and are doing it as we speak, so why wouldn't they do it again?
Microsoft seems to love having a premium and low end version of their products. They've been doing this for years not just with their Xbox line, but also their other products like Windows. They sell a higher cost "professional" version of Windows, and also a cheaper low end "Home edition" version. So this is something they seem to love doing and there's no reason to doubt they will do it again.
http://en.wikipedia.org/wiki/Windows_7_editionsActually, its worse than I thought. The lowest end one is "Starter" and there's a bunch more until you get up to the "Ultimate" edition.
But anyway, my point is this is their modus operandi, so not only should we not doubt they will do this with the next Xbox, we should expect it.