I have a little theory in mind , After hearing all of theses arguments.
First of all , Vergeben leak the Square rep in July/septembre (At least , way before the announcement for DLCs). The Square rep isn't in the base game , and well... Square won't do anything if they don't get more money than everyone else , so we DLC boys.
Then , the Plant. Sakurai stated that they got at little head start... But the model is finished , they were confident enough to show most of his moveset , in several stages against several characters. The only thing they can change are hitboxes and damage values , and it take way less Time than animations and making a moveset. To be simple , the Plant is almost finished , they started developpement earlier than most people think on her.
And then they announce DLCs , 1st november. They will be spread out from february 2019 to february 2020 (They said so in the Switch news feature (the one with the smash direct) ) and piranha plant will be available at least the 31 January , as stated on the Smash website if I recall.
Let's be honest , the first DLC must be BIG to convince people to buy the fighter pass and make a lot of money. I mean , they announced Mewtwo model back during Sm4sh and everyone was freaking out and hyped as hell. And it was just a model , imagine a trailer , even if It's a in-game on.
Except for MAYBE Katalina and Steve , the only DLC character leaked was a Square Rep.
Before saying "Nintendo choose the DLCs and the Smash dev Team haven't started developpement yet"... Well , why can't Nintendo ask for DLCs earlier than we think? Like , a little around when the Square rep was leaked , several months before the direct? If they are gonna spread out DLCs reveal , having a head start with the first one seem logical , especially if It's gonna be Big.
And well , Sakurai can lie to us , I think they prioritize the main game but got a smaller Team to work on DLC for now.
So , who do you think can be a Big first reveal , is a Square rep , and that the wider audience would know , while beeing again on the spotlight with a Mii costume , a Wii U eshop re-release , the SNES Mini , and social medias ?
Yeah , Mallow. Jk , Geno. ( I would love Mallow tho)
Sora and a DQ rep weren't really relevant or/and had a game coming to the Switch during this. KH3 can't Come to Switch except if it got a pocket edition like FF15 , and DQ release date for the Switch is... Well , even Square don't even know precisly.
They aren't Good candidates to start with because they are either less knowed worlwide , have less ties to Nintendo , and suffer of the "anime swordman" syndrome.
The only Square rep I could see getting a lot of people hyped (and beeing somewhat realistic) is Lara Croft but... Why? Why keeping her for first? It's only going to make people think they are going to go third party only , and isn't requested. She's a PS1 icon , but PS1 icons... Well let's Say most people would want Crash Bandicoot.
Getting a head start on for Geno as a DLC pick , a little after the plant doesn't seem impossible , ESPECIALLY if It's going to be the first reveal , wich is going to be at least february 2019 , 3 months After the game release.
Negociating earlier for him seem logical , since he is a third party , a good candidate for a first DLC (Requested , unique design that would appeal to people who don't know him , getting spotlight again theses 3 last years , can promote the SNES Mini and a SMRPG remake for Switch... The other Square rep don't have all of this at once.).
Add to that the thing that he the Square rep was leaked at the same time as Isabelle and Incineroar (BASE ROSTERS REVEALS MAY I ADD) by Vergeben , support this theory.
And now , PolarPandy has rumors about Geno beeing talked about in NoA and Nintendo made the selection for DLCs.
What else can I add ? The stars are aligned for him. Nintendo isn't stupid , he know he will make money for DLC and is way easier to get than other Square reps.
And since nobody can leak the Square rep otherwise they will be fired and sued , even if It's a fake name make me think that people who just flat-out said who the Square reps are liars. Square is very aggressive towards leaks.
I don't have anything else to add than:
-If he's in , I'm co-maining him and I'm gonna kick some butts in Online tourneys.
-If he's not , I'll still support him and I have the Mii costume , so I'll co-main Mii gunner lol