I think he'd be a nifty character to have. That's right, nifty. I wouldn't go on a huge campaign for him to be in it, but I definately wouldn't be angry if he did make it in. Plus, he could be like another Jigglypuff, everyone would get so pissed off if you beat them with Nook. Anyways, I saw a couple of points against him, so to argue:
1) "You may as well put a Nintendog character, or a Sims character!" - No, and no. They're not the same thing. Tom Nook has a personality, something a Nintendog does not have. Plus, a dog or a sims character is generic, Tom Nook is not.
2) "He's a shopkeeper!" - So? Who's to say shopkeepers can't fight? I mean, I'm pretty sure we've all seen those videos where someone tries to rob a convenience store, only to get the crap beaten out of them by the clerk.
And there was others, but I can't remember them.
Anyways, I think he would have a really unique moveset, I think one of his moves should be a furniture throw. He throws a leaf and then halfway through it turns into a random piece of furniture, couches being the most common, and then something giant like a christmas tree or something being most rare. It would be really amusing to whip couches at your opponents.