They'd have to add multiplayer to the mini-games suite, and maybe make the multiplayer stuff online. And then not sell it at full price.
And even then... it'd be a *maybe* for me. I really liked all the extras they threw into the DS game (and the "standard" game is classic of course, too) but... I've already played it. Nintendo would have to give me some other incentive to get me to buy the game AGAIN. Graphics aren't really gonna do it.
I think it's inevitable that Nintendo will eventually do a remake of SM64. If not for the 3DS then another system. I'm actually pretty curious as to what the new Mario game is going to be for the 3DS that Miyamoto was talking about recently. He seemed pretty excited about it.