Three Houses came out after Smash so I don't see how that was missing content, Fire Emblem did really well for remixes at launch too.
As for music, it's kinda hard to say. ARMS was produced by Kosuke Yabuki who also directed Mario Kart 8, and it's music was composed by Atsuko Asahi and Yasuaki Iwata who also worked on MK8 and several Smash titles. Sakurai would presumably have dealt with all 3 when implementing MK8 content in Smash 4 as well as Ultimate (There was several new tracks added to the MK stages). It seems to me that there was plenty opportunity for ARMS music at launch, suggesting that talks about how to represent ARMS started years ago, possible prior to ARMS' actual launch.
We assumed for the longest time that FF7 music remained as it was due to legal issues, whether thats the case or they held them off for the DLC is hard to say for sure, personally, considering it was only 9 tracks, I think saving them was probably wise. And besides, if they added tracks but skipped One Winged Angel that would have been pretty sus.
XCX's music could be missing for the same reasons, or it could be legal/cost related. I'd say at the moment it's looking good, the official soundtrack lists the music copyright as belonging to Nintendo and Monolith, through that says nothing to what sort of royalty plan they have set up with Sawano and his people. People often bring up Sony claiming the music on Youtube and suggesting they own it, but they're listed as owning the package of the soundtrack CD. It makes sense that they would claim pirated music released freely online for a product they make money from. And besides, claiming stuff on Youtube is a total mess.
Even assuming they somehow couldn't get XCX music in, and they had no plans to add Elma, I don't really understand why she didn't get a Mii Costume or Assist Trophy.
My hope is that they're just waiting to release her after XCX comes to Switch, which I personally think is very likely, as Monolith have been keeping XCX and even Elma specifically alive through stuff like her cameo in XC2 and her appearances in anniversary art for Monolith and Xenoblade. I constantly see comments online of people hoping for XCX to return so they can revisit it, or play it for the 1st time.