There are a lot of parallels in IX to stuff from Dawntrail, though it's often recontextualized to make it feel fresher and less predictable.
Wuk Lamat is the best of both Zidane and Garnet - Zidane's ever-optimistic attitude and willingness to help anyone for its own sake mixed with Garnet's sheltered royal upbringing and devotion to her people.
Zoraal Ja mixes Kuja's methods, Brahne's goals, and Amarant's personality. Post-Rite of Succession he empowers himself by gorging on souls and transforming with their power a la Trance Kuja, and hides feelings of inferiority (believing Gulool Ja Ja favored Koana and Wuk Lamat over him) behind a gigantic ego; his lust for war and conquest brings to mind Brahne's own ambitions of conquest; and his taciturn attitude and trust only in his own power are evocative of pre-character development Amarant. His weapon(s) of choice are ironically the same as Zidane's.
Ser Otis Velona is transparently inspired by Adalbert Steiner, Captain of the Pluto Knights from IX. Gulool Ja takes some cues from Vivi, being unusually erudite for his age and being a good friend with Otis (mirroring Vivi being the only person Steiner gets along with early in the game, and Steiner's protectiveness towards Vivi).
Sphene is Garnet recast into IX Garland's role. Both are artificial people created to preserve and restore long-dead civilizations, parasitizing other worlds in a futile attempt to stop the inevitable. Archeo Alexandria (found in the southwestern area of Heritage Found) looks strikingly like a ruined version of the same city from IX; when you run through it recreated during the Storm Surge in the final MSQ dungeon it looks even more like it did back in that game. It's important to remember that the Sphene we (quote unquote) interact with in Dawntrail isn't actually the original article, but a modified, memory-based construct created solely to love and protect the citizens of Alexandria. IX Garland serves much the same purpose for Terra.
Solution Nine is named after one of Zidane's Trance-only Dyne attacks. The fortunetelling building you warp in front of at the Neon Stein aetheryte features neon images of Yans (both the hostile ones and the friendly one) on its facade; these were the most powerful non-boss enemies in IX, with the friendly one being the final step to making Ozma more manageable.
The popcorn buckets in Living Memory are styled after the Gimme Cat, a fake friendly monster from IX. A Milalla (Lalafell / Dwarf) in Living Memory mentions his greeting of "Lally-ho!," his home of Conde Petie being destroyed in the Storm Surge, and Conde Petie being near the Iifa Tree (which the Aero Terminal is styled after); all of this is taken from IX, though the dwarves' greeting in IX was mistranslated as "Rally-ho!" and Conde Petie was destroyed when the Iifa Tree began to run wild after Disc 3. The Cleyra Museum of Nature references Cleyra, Burmecia's sister city built in a giant tree and protected by a sandstorm (until Beatrix stops the sandstorm and it gets nuked by a Brahne-summoned Odin). Asyle Volcane and the arena inside of it likely references Mount Gulug, which has something similar (where Kuja tries to extract Eiko's eidolons and you fight Meltigemini).
The final boss of Ihuykatumu, Apollyon, is a dead ringer for the Abaddon enemies from IX's rendition of Castle Pandemonium.
Tuliyollal being attacked almost immediately after Wuk Lamat (and Koana, I guess) are made the new Dawnservants references Kuja's attack on Alexandria the night after Garnet's coronation.
Lindblum, Alexandria's neighbor that set off the Storm Surge, has a counterpart in IX. The airship the real Sphene was riding when it was struck by lightning that implicitly killed her is patterned after Lindblum's air taxis in IX, albeit somewhat more high-tech.
That's just off the top of my head.