I'm not sure what they'd do with Mario. Unless they explored a different aspect of the Mario franchise altogether. How would one make a Mario hack n slash game? Perhaps if they...aha! A Bowser hack and slash game! The heroes and princess of the mushroom kingdom are ALL kidnapped and it's up to Bowser to drive off a greater threat. You can play as Bowser, Bowser Jr. or any of the Koopalings. They have their claws after all, and wands, and that paintbrush, and you could expand the characters further to give them each a fighting style. Bowser could turn into Giga Bowser with a powerup, and wreck shit even faster, the first canon appearance of Giga Bowser. And then the Koopalings and Bowser Junior could get Giga forms.

I mean...maybe...you know...that might work. Let's wait and see if Hyrule Warriors is any good first.

Tecmo Koei ain't exactly what you'd call poor. Their currently worth over a billion dollars and their actually worth more than capcom according to google finance ... 

how about dead or alive: volley ball featuring boobacious bikini wearing princess peach, princess zelda and princess daisy. My tits would be calm because that milk would be delicous

Oh my god, lol. I say let them!

Can someone tell me what game tecmo has that is really good? Tekken? Ninja gaiden is from them?

The thing is, nintendo could go and give then some of their subutilised ips like murasame castle, startropics, and other, but everyone just want to work with zelda and mario(in some cases pokemom). So, in the end, third party partners don't offer any variety to nintendo, and this is sad.

Super smash warrior.

I'd buy that day one.

I want to see how Mario Musou would be ha ha

I have a feeling Tecmo Koei doesn't really know what Mario is considering if they did, they'd know they wouldn't pull it off.

So they can ruin them games aswell?

How about no. They already ruined Metroid, lets keep it at that.