Well here's my idea for a first party pass, there are still many valuable characters from IPs to include and these inclusions are ones that I think have a decent to good chance, and they're also inclusions that I would like to see. I don't think we're actually getting a complete first party only pass but I'm entertaining this possibility anyways.
1- ARMS COMPOSITE REP
Nintendo being coy and mysterious with the identity of the character leads me to believe it's an ARMS character with multiple different alt skins. I believe Spring Man would be the default skin with Ribbon Girl, Ninjara, and Min Min as the other skins. I admit I'm not that knowledgeable on ARMS since I didn't buy the game, but from the gameplay videos I've seen, I believe it's possible to make these four characters fit with a composite moveset, at the cost of liberties taken to the characters. But Smash has always taken massive liberties when it comes to character portrayals, so I don't think that's a big problem. I'm interested in how the fighter might turn out to be, regardless of who it is it will definitely be a unique and interesting fighter.
2- WALUIGI
Welcome to the Wah zone. It's no secret that Waluigi has gotten a ton of mainstream attention and is one of Nintendo's most recognizable characters that isn't playable in Smash. This is the only assist trophy deconfirmation that Nintendo directly discussed regarding any backlash. Out of all Nintendo characters, very few have achieved this same notoriety. Does it guarantee him? No, but it sure as hell put him on everyone's radars.
Waluigi would be a really cool character to add to Smash bros. You can do different paths when it comes to making his moveset. You can go the spin-off path and make his moveset revolve around the items he uses in the Mario spin-offs. You can go the personality path and make his moveset revolve around his personality of being a cheating loser who will do anything to win. Waluigi is charming as hell and he's my most wanted playable character. I believe he is a fitting addition to the Smash roster and he has a good chance of making it in.
3- REX
Xenoblade Chronicles 2 is the best selling game of Monolith Software, exceeding all expectations from the company and from Nintendo themselves. Rex alongside Springman is a character that Sakurai said he would have added to the base roster if timing was on his side. Monolith Software has been a great partner with Nintendo and has even been tasked with developing BotW and it's sequel. Nintendo putting a character from Monolith Software makes a lot of sense, and my money is on Rex.
Rex would incorporate the blade system and he would bring Pyra and Mythra with him, the gameplay would have him change blades on the fly to fit the situation or to combo further with more moves. I'm hoping that if he gets in he gets more than just one blade, the potential with his moveset is immense and I don't want to see him robbed of that potential like Joker.
4- TOXTRICITY
Even though I'm not a fan of the new Pokemon game and it's direction, I can acknowledge a good Pokemon design when I see it. We may have gotten Gen 8 spirits as DLC, but they were of the first form of the starters and the legendaries, a ton of other candidates are still available from gen 8, including Toxtricity. I think Toxtricity has a really cool typing and design, and I believe this leads to an exciting moveset with this combination. They could go a ton of ways with this character, whether incorporating rhythm attacks or changing forms. Both require liberties taken, but hey, that's nothing new to Smash. Out of all Pokemon from gen 8 this is the one I'm most comfortable with.
5- ASHLEY
With a new WarioWare for the Switch being rumored, it seems it's not over yet for Ashley. The WarioWare series gained a momentary revival with Gold. Even though the 3DS was on it's way out, a new installment was released that performed better than a lot of 3DS games released during that year. WarioWare for the Switch has been rumored for some time now, and the series getting a push in Smash would help it make a comeback.
This character has a ton of potential to bring to Smash. Whether it's magic casting and potion crafting, or even using Red as a fighting partner in the moveset. I think Ashley would be a neat fighter to include from the WarioWare series.
6- ISAAC
A new trademark for the Golden Sun series has been filed and accepted recently, which leads people to believe that a new Golden Sun game might be in the works. Camelot also had no new games revealed so far which is interesting, as Camelot usaully had a new Mario sports game released in a two year interval between each other, which further raises suspicion on a new Golden Sun game being worked on.
Isaac would bring an interesting moveset to Smash that utilizes geomancy, something that hasn't been seen in Smash yet. Golden Sun has a lot of moves to offer besides using that big hand to slap people around in the assist trophy. So to see Isaac use his full potential in smash would be exciting to see.