New Pork City is a stage, yes, but in no way do I believe that should immediately label him as impossible. New Pork City is
the most well known area of Mother 3 except maybe that little sunflower area and Tazmily Village, but New Pork City had gimmicks inside for stage hazards so I'm assuming that's why they chose it over many others. Also, Porky appeared in more than one game. Besides, they made it back in
Brawl, I do not see how that limits his appearance. With the idea of "Everyone is Here!", it would make no sense to remove it and then bring it back again when it is supposed to be purely new content added to the game.
And there's one specific place in mind when it comes to Porky. As a matter of fact, it already came to light once before thanks to Smash Flash 2. I'm of course talking about the final boss area of EarthBound:
Devil's Machine.
You could also use Saturn Valley if you wish, but that's not particularly Porky themed if not just an EarthBound general theme. Devil's Machine is a clear hot ticket though, and Giygas' attacks and presence can easily be a stage hazard through unknown attacks and certain gimmicks.
Porky having a stage accompanying him is not impossible, so he's not ruled out.