So, honest question.
Which will get a new entry/new Smash character first? Which one will get one last?
Advance Wars
ARMS
Astral Chain
Golden Sun
Ring Fit Adventure
I have some thoughts on these.
Advance Wars...well the reboot is presumably still coming; as for a brand new game I think there is a solid chance if the reboot is successful. Unfortunately the delay will likely dampen the success of the reboot, though, so I hope Nintendo takes the circumstances into account when evaluating how it does.
As for a Smash rep, I have long been hopeful for it, but likely the series needs the brand new game beyond just the remake, and probably also that new game needs to be released in and successful in Japan, which even I'll admit is a relatively tall order when the reboot camp might not even release in Japan AFAIK.
I've talked about ARMS in here recently, so I'll just reiterate that it probably needs a sequel to get a second rep and that said sequel seems relatively unlikely but certainly not impossible. ARMS fit its bill as an early-lifecycle proof-of-concept of sorts for the JoyCons; I don't know that it was successful enough to grow into more than that but at least it has a lovable cast to work with if they wanted to expand on that universe.
Astral Chain, if it got in, would likely be on the lower-priority end and would likely require us to get a large number of newcomers on the scale of 4, in which case it could potentially fit into a Shulk-type roster slot as a niche but still reasonably recent first party series. It sold over 1mil, which is great for a new IP, but in the context of the Switch's large install base it's not over-the-top impressive relative to the performances of other IPs on the same system. My guess is that Astral Chain may very well fall into the vein of many other first party series that had comparable levels of success but didn't get into Smash or get sequels for multiple console gens. I don't think Nintendo buying the rights is a definite indication of a sequel, either. So overall I'm leaning towards it simply being outclassed by other series that have had some combination of bigger fan demand to come to Smash, bigger sales, and/or a longer history/library of games.
Golden Sun, as I have said before, has a window right about now to get a new game. It's the later part of a console's lifecycle with most of the bigger franchises already there and, crucially, Tennis/Golf already done. AFAIK we don't know what Camelot is working on next and as someone else pointed out there was the Camelot website update a while ago, too. So something could be up soon for GS. If not, however, then I think once we get to a new console the chances go down, as likely Nintendo would want new versions of Tennis/Golf for the next console gen. If a new platform is coming soon, then GS's best chance would be for Golf/Tennis to simply be ported to the new hardware so that Camelot can do something else; however, that didn't happen for the Wii U versions on the Switch so that scenario seems unlikely.
As for Isaac in Smash, a successful new game would probably make it a slam dunk in combination with Isaac's consistently high demand and strong ballot performance. Without the new game, Isaac's relying more on the demand and/or the possible existence of a post-1980s retro slot to make it in; still easily possible but his chances depend on how he stacks up against certain others like Waluigi, Dee, etc. (personally, I think he still compares favorably).
Ring Fit seems like a strange case. It did really, really well but you can see circumstances in which it wouldn't get a character or a sequel.
In considering a sequel, remember how successful Wii Fit was and that it didn't continue on the Switch. Almost certainly Nintendo will continue making unique exergaming experiences but we don't know if that will take the form of more projects under the Ring Fit branding or something else entirely new; personally I lean toward the latter, as it feels like Ring Fit's success is predicated more on Nintendo's adeptness at putting attractive spins on the genre than it is the Ring Fit branding specifically. Still, you can't count it out given the success level. I guess a more positive way to put this would be "well, if it doesn't get a sequel then it's probably getting at least a spiritual successor of sorts."
As for the character, it could happen regardless of the sequel. I would put it above the other Switch-only IPs like Astral Chain and ARMS due to the significantly higher level of commercial success. Probably ahead of AW, tossup with GS in terms of likelihood.