Being new has very little to do with getting added, I don't know why people think it matters so much. Sure, Sakurai wants to include new series if he can, but those series still have the same standards to meet as all the others, it's not like being new is a boon. If anything, new series seem to be judged fairly hardly by Sakurai, look at the three (and only three) we've gotten since Smash started, Pikmin, AC, and Wii Fit. These are pretty big series, who are comparable, or even bigger, than most of the older series added. Sure, every time the bar lowers a bit in terms of standards to be met, but... the standard for inclusion still seems to be pretty lofty. Now, considering we really don't have a precedent of Sakurai adding series based on future potential as much as what they've already done, I'd say Shulk's chances, while definitely notable, are still pretty questionable.
If any series post-Brawl is to be included... there's a good chance it'll be his. But the thing is, Xenoblade, as it stood when the roster was being compiled, not what it might be able to accomplish in the future, was still one game that might've made a loud splash, but didn't make a big splash. Of course it's a great game, and personally, I would really like to see Shulk and lots of Xenoblade (and X) content, don't get me wrong, but its overall impact was, compared to other series still unrepresented, more on the minimal side. In the west, Xenoblade didn't bomb or anything that drastic, but it's not like it can claim to have done well, despite what vgchartz (which isn't accurate) would have you believe. In Japan it was outsold by The Last Story, which also didn't do phenomenally. Shulk is a hot character among our demographic, but overall his audience is rather small. Now I'm fully aware sales don't equate to inclusion, but they are an indication of rough audience capacity, and size of audience that would recognize the character does matter as far as inclusion is concerned; Sakurai goes with the biggest series, the ones that have left the greatest impacts.
Even if less recent than Xenoblade, the fact that Golden Sun has had an audience... in the millions, which is several times more than Xenoblade, (just by sales it's roughly five times more), it would indicate that chances are Sakurai would opt for its inclusion first. The two series might seem closer to equal from our perspective, but out there in casual land, where the majority of the sales live, it's much more skewed in Golden Sun's favour. While GS is obviously not at its peak anymore, pretty much the same could've been said for FE, Metroid, and Mother when they were included. FE was on a descent much like GS, and the latter two hadn't even received games for half a decade. But they were still overall important to Nintendo, and their impact still resonated enough to wind up being included.
The age of the series doesn't matter to Sakurai (excluding retros ofc, who are obviously in their own category) nearly as much as the impact and popularity the series has had. One day the Xenoblade series (if it indeed does turn into a series) might be in a place where its impact is comparable to GS's, but it isn't yet, and it wasn't when Sakurai was compiling the roster. Especially since NoA won't acknowledge Rainfall as an influence and Japan wasn't even affected by the impacts of the campaign.
Takamaru has a better chance then both though, yeah. Though not for the "quadruplet" reason.