Alright, question time so it doesn't seem like I'm ignoring my own thread
Aside from Agumon, are there any other Digimon you'd like to see as a fighter? The bulk of my experience comes from the first three animes (Adventure, 02, Tamers), but I remember thinking Guilmon was pretty cool. Funnily enough, I believe he was originally made in-universe as someone who's "cooler than even Agumon."
If Agumon is "the Pikachu and the Charmander" as some folk call it, then the alternative potential standalone fighter is Omnimon who is "the Charizard and the Mewtwo". They are the result of Agumon's most iconic Mega Level form and Gabumon (the Eevee and the Squirtle)'s most iconic Mega Level form fusing.
They are such a big deal that they were the first chase card of the new TCG and have already had a full set named after them. They also play a huge role in Cyber Sleuth. They are also the leader of the Twelve most iconic "Legendary" Digimon and the right hand of Yggdrasil, The World Server, the very mind of their reality.
If any Digimon got in over Agumon it would be them; they are such a big deal that if we get Agumon they single handedly max out the odds of Gabumon being the Bonus Fighter just so that if the two of them were on a team together when one of them got their Final Smash they would be able to fuse.
To the point where I have sorta theory crafted a version of my "branched evolution" gimmick for Gabumon.
Also they would allow for a really cool reveal scene; you see the have a - for a Mega Level Digimon - relatively simple form but one which is very relevant to Smash Bros.
One Arm is themed for War Greymon and has the ability to generate a Sword from the War Greymon head it has instead of a hand. The other Arm is themed for MetalGarurumon and has the ability to generate a Cannon from the MetalGarurumon head it has instead of a hand. Essentially meaning they are all three Mii Fighters styles in one.
And Digimon is on some level about software animism (and the effect of information technology on the occultist framework derived from combining ideas of Plato's World of Forms and collective consciousness if they can be applied to our technological psychic network of machines in a similar way).
Essentially it assigns a certain spiritual reality to (a lot of things but most importantly for our purposes) video game content.
In the most iconic Digimon movie Omnimon's body is formed for the first time in the franchise from the hopes and strength of kids the world over, all providing him their power the way a Tamer would; in order to stop a Virus Digimon who hacked the Pentagon from nuking Japan and starting World War 3. - If that premise sounds sorta familiar, Summer Wars is a spiritual re-imagining of Digimon's Our War Game.
So what I would love to see if they got Gabumon as a bonus fighter, or just went with Omnimon is based on that scenario.
Red becomes Agumon/WarGreymon's Tamer, Leaf/Joker/Legendary Sinnoh Trainer/Demi-Fiend becomes Gabumon/MetalGarurumon's Tamer. If separate they just did their individual Final Smashes but it couldn't turn the tide.
They are beaten down, barely alive. Joker (or whoever) and Red plead with them.
"I don't have some DigiFlute to wake you, but I want you to know you're not alone, okay? And reinforcements keep coming, they won't stop. Heroes from all over the multiverse are fighting for us; we're the only ones who can do it."
A panning shot over the battle shows different Mii Costume characters.
"Feel their hope, feel their strength."
"I feel them."
MetalGarurumon and WarGreymon fuse together, as the characters used for Mii Sword Fighters, Mii Brawlers, and Mii Gunners form their body.
From the battle field Sonic takes notice and draws Mario's attention.
"Whoa, Part WarGreymon..."
"Part MetalGarurumon!"
"They're Final Smashing together to become -"
The new hero decrees his name.
"Omnimon!"
Omnimon then decimates the army with their Transcendent Sword and obliterates Dharkon with their Supreme Canon.
(Despite a lot of people remembering that movie fondly; sadly the dubbed version is generally regarded as the most butchered piece of dubbed Digimon in a franchise famous for generally not doing that to its anime adaptation at a time when that was the norm... and yet despite being three Digimon movies crammed together with a short for some dumb other show by that production company shoved into it, it still kinda works.)