How hardcore? If you’re talking ‘fast in rough terrain’ and I were building one I’d be looking at the new trek stache with normal 29er wheels and 140mm forks. Crazy short chainstays, massive tyre clearance and tensioner if you want to make it really hardcore and go single speed. And if you got bored of that you could turn it back to a 29er plus for more of an all-rounder.
Funnily enough, this is more or less what considering for my next bike.
Also tried a number ros9 a friend had on demo which was good fun. Less versatile though.