1) Assist Trophy
I want to start by pointing out how odd it is for Advance Wars to go without it's AT in the game. One might pass it off aside as nothing but you have to consider it a bit more than that.
Advance Wars/Famicom Wars, while not a "major" Nintendo franchise by any means is still one with great historical significance to the company and has it's history going back all the way to the NES. A series that spanned just about every Nintendo platform prior to the Wii U/3DS generation. In addition to that, the game has had a great impact on the way Nintendo viewed turn-based strategy games. Prior to the release of Advance Wars, Nintendo thought the genre would never be successful until their marketing team played it and loved it, asking why they couldn't release the game overseas outside of Japan. It was the success of this game that gave Intelligent Systems/Nintendo the confidence to release Fire Emblem outside Japan and paved the way for that franchises success and you can see where it's at today.
So based on what I mentioned above, this isn't a series you just discard to the wayside. Advance Wars, while a niche series is still very important to the company's history going back 30 years. Consider all the other Assist Trophies that were added in to this game or returned from previous games. A good half of them have even less relevance than Advance Wars does and/or are more obscure/niche than said series.
You can say "well the Infantry & Tanks Assist Trophy wouldn't work well with AT's being KO'able" but even with that, it's not a series Sakurai, a man who loves the history of gaming and celebrates it through this game, would throw to the wayside like that. Knowing Sakurai, he'd give the AT an overhaul before doing away with it like that.
And if the compromise was really not having any Assist Trophies, I believe Advance Wars would have gotten a Mii Costume like fellow dormant franchise Custom Robo (which by the way is even more niche and has been gone longer than Advance Wars has, no disrespect to CR fans). It didn't get that either. Sakurai is not one to do a franchise like that so much injustice.
But of course, the series wasn't completely absent and it has a presence in the new "Spirits" mode. Another thing which I found very interesting mind you based on a lot of things I picked up on so let me elaborate...
2) Spirits Mode
So as we all found out in the Direct, there's a new mode called "Spirits" in this Direct that serves as a replacement to Trophies and are like glorified Stickers from Brawl (albeit actually interesting lol).
I pointed out earlier that two characters from Advance Wars were shown off in that mode already (with Sakurai hinting at a huge amount of Spirits still left to be discovered once players get their hands on the game).
These two characters being Eagle and Olaf, CO's of the Green Earth and Blue Moon respectively. On a minor note, I picked up on the fact that Sakurai seems to be spanning different games for the CO's as Eagle is based on his AW1 artwork and Olaf on AW2. While it's nice to see the series getting some love in the game through Spirits, there's likely to be a lot more shown off if we already came across 2 spirits from that series as well. I'm thinking maybe we get at least all the AW1 CO's in this mode as Spirits.
But, oddly enough, that wasn't even the biggest take away I got from all this.
After watching the Direct multiple times, I caught on to the fact that Advance Wars is seemingly the only Nintendo franchise (not represented with a playable character) that doesn't have it's main character represented in Spirits. This might go by some as not being much of anything but it's still an important thing to factor in. I guess I should point each out...
Ray from Custom Robo
Saki from Sin & Punishment
Lip from Panel De Pon
Dillion from Dillion's Rolling Western (also an Assist Trophy)
Matthew from Golden Sun (in addition to Isaac being an Assist Trophy)
Chrous Kids, Rhythm Wrestler, and Karate Joe all from Rhythm Heaven
Savvy Stylist (Hope I got the name right lol)
Diskun from Pushmo
Starfy (Titular Character)
The Ever Oasis Protag (the name escapes me lol)
Henry Flemming from Codename Steam
All of this is addition to Springman/Ribbon Girl who were deconfirmed as an Assist Trophy/Mii Costume respectively and Nintendo LABO which was soft deconfirmed as a Mii Costume
Sorry for posting just about every Nintendo franchise not represented but I felt that I really needed to drive the point home. Literally every series other than Advance Wars is represented with it's main protagonist in some shape or form. If there is some series then they're from the most obscure Nintendo franchises or really retro.
I feel this is way too convenient to be passed off as a "coincidence". Just take a look for yourself. Especially for a series that's already confirmed to have two spirits in game and plenty more to come in the full game most likely.
So why is it that Andy, the mascot for the Wars series as a whole is completely absent from spirits, being an AT, or a Mii Costume when
LITERALLY every Nintendo franchise without a playable character was shown to have one of those things? Especially when series much more niche were shown off right off the bat to have their character in as if Sakurai wanted to deconfirm the possibility of them being playable in game right away? Why wouldn't he do the same for Andy if he wasn't going to be playable?
Just saying it's a coincidence isn't substantial. There's a lot more to this especially in the wake of Advance Wars being absent in every other area other than Spirits.
3) The Rumor from Last Year
While this is arguably the weakest point in support of this theory, it's worth mentioning that there was a
4Chan leak a while ago that had Advance Wars 3DS on it and it's also the same leak that predicted Wario Ware Gold (the exact number of micro games mind you).
Granted that leak looks to be more and more false as time goes on, I think it's worth noting that if a game like that were to happen, it could definitely give Advance Wars a shot at Smash DLC even if said title isn't on the Switch. Something to take in to consideration if nothing else.
Conclusion
That's my big theory on why I think Andy might have a chance at being DLC with the Infantry & Tanks incorporated in his moveset. Like I said, a lot of this can be perceived as "reaching" by some but there's a lot of points I made that are worth noting and taking in to consideration.
I guess the big counterpoint against all this that Advance Wars wouldn't have a shot at DLC because of it being obscure and to that I say it's the moveset that sells Andy as a character rather than the brand awareness, what have you. It's a moveset that Sakurai can sink his teeth in to and one never seen in Smash Bros before. A fund based system where players can purchase units and have them fight for them would be a lot of fun for players not even familiar with the Advance Wars series. And like every other niche series that got a huge boost through Smash Bros (looking at you Fire Emblem & Kid Icarus), Advance Wars has much to gain from Smash inclusion and would naturally lead to a revival for the series taking it to levels it's never been before.
Ahh well, here's hoping.