What franchise do you think would be a lock for a Smash inclusion if they got one recent game?
I think everyone else has been pretty spot on. Although it's worth mentioning, these games would have to show up pretty damn soon for me to be convinced. Sakurai could get tipped off on it early where it wouldn't be an issue, but if Smash is getting plotted out right now these might end up missing the boat. In any case, ignoring that...
Golden Sun, yeah. Isaac is already in a pretty good place right now as a popular request, and if Golden Sun suddenly became a relevant series again there would be pretty much no reason to exclude him. Except for not being the protagonist, which... yeah, I don't think it's a problem. The fan demand and seniority would probably negate this and Matthew or whoever else may be able to slot in as an echo fighter. Easy.
Rhythm Heaven, maybe. I would hope it at least has them considering better supplemental content, but I wouldn't be 1000% convinced on a character. I have to wonder if Sakurai passed on them for a reason, and unlike Golden Sun it's a less clear cut thing who they would add in the first place. I do think it would increase their chances exponentially, but I wouldn't quite put my complete faith in it. Even though I'd like to... since yknow, Chorus Kids are my most wanted. Worst case scenario, just give us a stage and some more music.
Murasame Castle, yep. If this got rebooted and released in the states, there would be nothing stopping Takamaru. I get the impression Sakurai likes him a lot - enough to have an explicit reason that he passed on him, which means he was considered - and Nintendo holds it in fairly high regard as a legacy title. I consider this comparable to Punch-Out Wii's effect on Little Mac, while I still think Mac would be on the table I'm convinced it must have put him over the edge to have had a recent game under his belt. This may be able to apply to any "retro" series but Takamaru is both the most notable example and probably the one most likely to happen.
I think any series getting a revival or new entry in the next couple years would put it in the conversation really, and it would be for good reason. Outside of Ring Fit and occasionally Astral Chain our pool of potential new IPs is resting on either popularity or blind faith. So any of these popping back up would theoretically serve them well. Advance Wars is a weird case where this did happen, but the game never even made it to Japan so I wouldn't put much stock in that. Speaking of Astral Chain, I'd say a second entry in that series would have me taking it more seriously. I'm not convinced just yet, although Ring Fit does have a great chance because of how exceptionally well it performed.