ARMS is barely a year old. The only times we've seen new characters appear despite their age were Roy, Greninja and Corrin; the first one is unique in that he is the only character ever simply for commercial reasons, which means it isn't a situation that will occur frequently, and is an exception by itself. Greninja was added essentially out of tradition,
said by Sakurai as the only time a "slot" is saved, not even having a specific Pokemon in mind. Corrin is similar to Roy, and actually might be the best example to support why ARMS will receive representation. Corrin was chosen because they desired a character from a game either newly released or soon to be released,
and Fates just happened to be that. That said I don't think that's fully true, as Splatoon, despite is unforseen popularity, fits the bill (even if it is a bit older than the time frame Sakurai's team wanted). In terms of other content like stages, it's more possible, but even then examples of a new game getting a stage is few, with the most notable example being Wooly World, which was added to replace a Kirby's Epic Yarn stage so it had a previous iteration beforehand.
Palutena and Bowser Jr. aren't really good comparisons. Both are from already represented franchises, the former from Sakurai's personal baby and the latter from
the main franchise.
Furthermore, it's actually appropriate to look at one of the newcomers: Inkling. Inkling sports her Splat 1 design - from the gear all the way to the weapons and moveset. Splat 2 is only a month older than ARMS, and they could have easily focused on updating the design and moveset to add in anything from Splat 2. Now, I can see the argument that Splat 1 released even before DLC for the game ended so they started working on it in the two years before Splat 2, but again, I'd like to emphasize the similarity in time with ARMS' release, as well as the huge popularity of Splat 2.
I think it's more appropriate for to say that I'd be both surprised and unsurprised if ARMS gets more than Trophies, or even a stage. The game isn't even the smash hit that Splatoon was, which if it was I'd consider their chances more.