If they wanted to do another "plant" then why would they save it for last instead of putting him first.
Simply because it would be the best moment to add the new Plant. Piranha Plant is a DLC fighter, sure, but at the same time is an exception as it is not part of any pass, and arguably was the finishing touch of the Smash roster. Never planned to be in the base roster, but always planned to end the base game and start the DLC part (as shown with it being the only DLC choice that has a Palutena Guidance). Genuine leakers also told us it was already playable back in September, so it does reinforce this idea they had it planned for a long time already.
As to why put Porky last? Because it's the sport that's, usually, reserved for less "hype worthy" characters for the general population. Actually, it's not usually, it's always the case.
Melee ended with Roy and Game & Watch. I'm not hating on the characters, but one's game was delayed thus making him representing a non-existant game, and G&W is an extremely surprising choice that I don't think anyone really wanted.
Brawl ended with ROB, Toon Link and Wolf. While not "bad" characters, Toon Link and Wolf are two of the semi-clones, and were almost not added to Brawl if Sakurai didn't get an extra delay to include Sonic (Jigglypuff would also have been cut, by the way). Meaning that, really, ROB was planned to be the final hidden character at some point.
Smash 4 ended with Duck Hunt. I'm probably one of the only ten human beings on this planet that wanted the dog, but frankly, I didn't see much support for Donald and Goofy.
Smash 4's DLC ended with Bayonetta and Corrin. Frankly not the worst outcome, but it's still another Fire Emblem character that's already argued to have too much representation, as well as a third-party character who is clearly not on the same scale as Cloud and Ryu.
Ultimate ended with Incineroar, Ken and arguably Piranha Plant who were all revealed the same day. Again, I don't hate the characters but if they intended on making a grand finale, K. Rool would have definitely worked better, but Sakurai for X reason decided on finishing with another Pokemon and the plant.
FP1 ended with Byleth, same issue than Corrin. Despite the first pass having notable third-party fighters the whole time.
I'm sure the big names for the second pass were Sephiroth and Steve. I don't expect an exceptional finale. Maybe it won't be Porky, sure. But I don't see a Crash Bandicoot or Doomguy either. However I stand by my point that Porky's best shot has, and always has been, the final slot, only if it's revealed in September.
If they wait until TGA, welp kiss good bye to him lol