I think it should be mandatory thatthey have the ability to allow for tablets, but these days I do not think they need to. Now if it is billed as a casual system again like the wii was then t will need to allow four tablets. But do that many people play multiplayer in the same room anymore? If they do is it more than two people at a time? There are a few games where the more you have the merrier! But even on a big Tv 4 screens starts to get a bit annoying. As long as Nintendo has good Internet two controllers should not be too big of deal. I think if they can allow two tablets and two motion control or even four motion control then that would he a viable option as well.
Well the best selling Wii games often included 4 player support. We regularly play with 4 or more people (taking turns) and most of my friends bought a Wii at some point because of the multiplayer games. I'd say it actually is the biggest selling feature of the console.
A Wii U with 2 player only support would... fail. And I mean fail as in Nintendo will look back at the Gamecube and say "man those were the times."