International companies cannot buy Japanese ones.

Or the people that already aren't fond of Mario are like wtf is he doing in my game.  Or Nintendo fans are like why is Drake in SSB. I don't think they have a single, for lack of a better word, "realistic" character on the roster.

Sony doesn't really need mascots it has Naughty Dog and kills of almost each succesful franchise after a decade, Nintendo lives on their mascots. Mario on a Sony a console is not a big deal for Sonyfans and Hardcore nintendo fans probably think those are lesser characters anyway. Third party and console exclusive trade ins work better and are way easier.

That rules out MS, but Japan still has Square, Bandai Namco and Capcom

No they shouldn't. That would be stupid.

What Nintendo and Sony should do, something that would be incredibly intelligent and forward thinking, is allow for cross-platform online play between the PS4 and the NX. Any multiplat with online multiplayer is playable with each other, whether you're on PSN or the NN. Android and iOS already do this.

Having these guys do it as well would be far more beneficial to both companies.

I think that's on a too small of a scale to do anything for either company. Maybe co-developing some JRPGs or something might work better.

They should partner with MS and take Rare's IP's and actually use them.

Why would third parties bring their games to a Nintendo console they didn't sell well on recent Nintendo systems unless they are exclusive. It would be benneficial for Nintendo perhaps but I earlier see an allience between Sony and MS than between Sony and Nintendo. Mostly becuase Nintendo will only allow cross party at their (2.8 trillion) rules.

Because I think NX will actually target the audience that buys 3rd party games this time.

I earlier see an alliance between Sony and MS than Sony and Nintendo as well, but this thread clearly isn't about likely scenarios. It's about beneficial ones.

Well it better launch with a real Metroid, a new M rated IP and F-zero in that case and is a graphical powerbeast of €400- in that case so it will send out  clear message.