Someone right above my post already said most of my points, so I'll just one I didn't see
Most notably the fact that Nintendo and Sakurai can just do whatever the hell they want with Smash because it's basically their thing. If they want to promote an Assist Trophy through playable means, there's really nothing stopping them for any DLC that's decided after the base game since it's beyond their initial plan.
Heck, it wouldn't even be the first Nintendo fighting game where an Assist-type character gets promoted to playable, as that honor goes to Croagunk in Pokken. So there's definitely precedent to this idea of a promotion and if that's what they want to go for, they'll just go for it, end of story.
Should also point out Croagunk is the only example from Pokken, so in other words, if we
do get an Assist promotion in the second Pass, don't expect it to be a norm. It would likely be the only case for the entire wave.
What makes me think it
won't be Spring Man is already said above; that guessing game would be utterly pointless if we got the most obvious choice.
It'd be like if, back in 2015, Nintendo said "we're getting a fighter from Street Fighter II, but we're not saying who" when pretty much everyone could tell in advance that it'd be Ryu, and not just because it leaked through datamines, but because Ryu is the mascot for the franchise, if not the entire fighting genre. Spending weeks of making us guess would be a complete waste of time because we'd figure Nintendo out in one second, if even that.