My predictions, in no particular order:
FP2-1: Ryu Hayabusa. There’s a rumor that Byleth was originally gonna be in pass 2 but got expedited to pass 1 cause Hayabusa was leaked. Hayabusa is a common name among insiders while almost NO insiders knew of Byleth.
FP2-2: Neku Sakuraba. The The World Ends With You site went under a surprise maintenance lately. Last time this happened was before Final Remix’s release, and I doubt we’d see a sequel just yet. Nintendo’s Twitter accounts also heavily promoted Final Remix before its release. I also doubt Geno will make the jump from Mii Costume in 4 to playable in Ultimate via DLC. See KOS-MOS’s section for my reasoning.
FP2-3: Elma. First Spirit promotion. Rex was shown as a Mii Costume at the start of Pass 1 and apologized for him not being playable. It could be that ANOTHER Xenoblade character is planned, especially if Xenoblade Chronicles X Definitive Edition becomes a thing. Plus, Caitlin Glass’s NDA is STILL unknown. CRAZY STRAW GRASPING - also, Fiora was added to Shulk’s Chain Attack Final Smash despite Chain Attacks in 1 and 2 having only three party members. X, however, has four party members and Overdrive instead of Chain Attack but that’s not important so Fiora breaking the three members only rule may be a reference to X. Again this here is just a stretch but I can see it.
FP2-4: Octoling (Splatoon 2 Octo Expansion). Second Spirit promotion. At the start of the year, something new involving Splatoon 2 was teased months after Final Fest (the SAVE OUR SALMONS thing). Octo Expansion was a huge hit and Splatoon 2 is still played widely today even after Final Fest. A major Splatoon 2 update is also scheduled for April. And despite Rex being a costume, Spring Man an Assist Trophy and Officer Howard from Astral Chain an Event Spirit, Byleth proves there’s no such thing as “too new” for Smash. Octo Expansion came out in 2018, and Inklings can be used as a base for an easy Isabelle style semi-clone if not Echo.
FP2-5: KOS-MOS. She was deconfirmed for the fifth pass character, sure, but the second pass opens up an opportunity. And before you say Heihachi or Lloyd, it’s very unlikely a 4 Costume will be promoted to playable in DLC. So far that’s only happened in base with Inkling, Isabelle (who was also an AT in 4), Chrom, King K. Rool and I think I’m forgetting one more. Maybe not. But my point stands and while a Xenosaga HD Trilogy was deemed financially unfeasible, the interest is very much there. Plus, KOS-MOS appeared in Xenoblade 2 as a Blade alongside Shulk, Fiora and Elma; and in the Project X Zone series with Ryu and Ken (and in 2, Chrom, Lucina and Fiora).
FP2-6: Arle Nadja. Why DID the Puyo Puyo director retweet the 1/16 announcement when Byleth was revealed...? Arle wasn’t even a Mii costume. Unless... Arle will be in Pass 2. Arle was another big name among insiders, and since Lip from Panel de Pon couldn’t be added as playable, Arle may be the next best thing. Plus, Sega has been promoting Puyo Puyo like CRAZY as of late.