The lack of X content so far could just be a coincidence. Lots of series haven't had much focus yet, like the dozen or so that don't have a song on the website. There's no more withheld X content then there is withheld Kid Icarus content, for instance. They might just not be seen as important enough to be put up on the site or featured in a direct. If Sawano is indeed composing for Smash as his earlier project then we'll probably get a song of his before the game's out but that won't necessarily be XCX related either.
At least the rumours saying we're getting two directs mean we might see some new X content be showcased.
Here is a little list of new content from games that released after Smash For's project plan for every series featured:
-Super Mario: Odyssey stage, alts and music tracks.
-Donkey Kong: Nothing so far (but most likely because the focus before release was on King K. Rool, I'm certain we will have songs from Tropical Freeze).
-Zelda: BotW stage and Link, ALBW Zelda.
-Metroid: Meta Ridley is similar to [/spoiler] his Samus Returns appereance [/spoiler] but also there weren't new games because Samus Returns was in early development.
-Yoshi: Yoshi's New Island Remix
-Kirby: Nothing yet, but it could be tied to either Bandana Dee being in or the infamous Sakurai's bias towards his own Kirby games (but the series did get a new City Trial remix and Chef Kawasaki as an AT).
-Starfox: The designs are from Star Fox Zero.
-Pokémon: We have heard some remixes from Sun and Moon, and many of those Pokémons are now inside Poké Balls.
-Earthbound: Nothing so far, most likely due to lack of new games.
-F-Zero: A new remix from an old game because of lack of new titles.
-Ice Climbers: Nothing due to lack of new games.
-Fire Emblem: Fire Emblem Heroes remix.
-Game & Watch: Nothing due to lack of new games.
-Kid Icarus: The Staff item, but otherwise no new games.
-Wario: Game & Wario was already featured in Wii U, and WarioWare Gold was made too late in development (and it is more of a compilation rather than a new title) so no new games.
-Metal Gear: TP are a special case since they go with what is iconic rather than what's new, but there is a new Snake Eater remix.
-Sonic: See above (and we also know that it has some new songs).
-Pikmin: Pikmin 3 was already featured in Wii U, so no new games.
-ROB: Nothing to say.
-Animal Crossing: Isabelle is playable and there is an Happy Home Designer remix.
-MegaMan: See what I wrote about TP under Metal Gear, but it has several new remixes.
-Wii Fit: No new games but there is a new remix.
-Punch-Out: Nothing so far due to lack of new games.
-Mii: It's not really a series by itself but there is a new Tomodachi Life remix.
-Pac Man: See above about TP.
-Xenoblade: Nothing so far except Fiora in Shulk's FF
when there have been 2 more games (XC2 was in early development when Ultimate was planned).
-Duck Hunt: No new games but there is a new remix.
-Street Fighter: See above about TP but there are new remixes.
-Final Fantasy: Nothing due to striking just to VII and licensing issues.
-Bayonetta: Bayonetta 2 was already featured but Rodin is a new AT.
-Splatoon: Besides all the new contents from the first game Lil'Judd from Splatoon 2 is on Mooray Towers.
-Castlevania: See above about TP but the Vampire Killer's design is based on the Combat Cross from the Lords of Shadow series, which to date are the latest Castlevania games.
So far every series has received new content from their latest entries, with the only exception being series with no new games (or too late in development), TP series due to "iconic over recent" and Donkey Kong most likely in order to promot King K. Rool.
Xenoblade (and to a lesser extent Kirby) are the only series with several new titles since Smash For that have yet to receive any new content from said games, and there has to be a reason for keeping it a secret, since when you include stuff from Happy Home Designer and Yoshi's New Island you can't just overlook a big title such as Xenoblade Chronicles X.