Dangling Plot Threads

As in, what is X that was just name-dropped, or when will Y come back and finish its story?

1.X (Mondstadt/Liyue)

[X] Arcadia (1.0, Kaeya SQ) - name-dropped in 4.0 (Ann of the Narzissenkreuz); mentioned in "6.5" Luna-VI WQ (Dornman Port)

[X] Sinners (1.0, Kaeya's voicelines, More About Kaeya: IV and Chunk of Aerosiderite) - resolved in 4.7 (AQ) and shown in "6.1" Luna-II (AQ)

[X] Childe's judgment (1.1, The Golden House Trounce Domain) - resolved in 4.X and reputation among colleagues restored in "6.3" Luna-IV (Arlecchino)

[X] The false sky (1.1, Unreconciled Stars) - shown in 5.2 (AQ), 5.7 (Skirk Character Anecdote; Skirk SQ), and "6.3" Luna-IV (AQ)

[X] The Heart of Naberius (1.2, Albedo Character Story 4) - resolved in 5.6 AQ Paralogism

[?] The borders of Teyvat (1.4/2.3, Wings of Feasting) - mentioned in 5.7 (Skirk SQ about Alice), "6.2" Luna-III (Alice)

2.X (Inazuma)

[X] One-horned white horse (2.2, Cinnabar Spindle) - resolved in "6.1" Luna-II (Rhinedottir is represented by a unicorn chess piece)

[_] Rhinedottir's Primordial Human Project (2.2) - mentioned in 4.2 ("Notes" interactable readable found in the Narzissenkreuz Ordo); Primordial human mentioned in "6.5" Luma-VI WQ (Dornman Port)

[X] Reborn Hydro Dragon (2.3, Before Sun and Moon) - resolved in 4.X (AQ & Fontaine glider)

[X] When the Doves Held Branches (as in, why doves?) (2.3, Before Sun and Moon) - resolved in "6.3" Luna-IV (AQ)

[_] (Breaking) the "eggshell" (2.3, Before Sun and Moon) - mentioned in 4.2 (Neuvillette teapot line, as in breaking the eggshell)

[_] The inverted fountain in the Chasm (2.6, AQ) - Also appears in the Book of Revealing (4.0-4.2)

[_] The "chaotic space" in the Chasm (2.7, AQ)

3.X (Sumeru)

[_] Giant tree (Sumeru? Irminsul?) in flames (Sumeru Promotional Video) - brought up again in "6.6" Luna-VII version trailer

[X] Slumbering Heavenly Principles (3.2) - resolved in 4.X (Neuvillette: Vision)

4.X (Fontaine)

[X] Golden Hyperborea (4.0, Ann of the Narzissenkreuz; Lumidouce Bell description) - resolved in 6.X

[_] Ancient City of Ys (4.0, Ann of the Narzissenkreuz)

[_] The wishing menu/circle of clouds (4.1 Neuvillette's Character Demo)

[_] Aqua Doloris (only because I'm convinced it has something to do with the Dark Sea) (4.1 Wriothesley SQ)

[_] The 3rd Descender and the gnoses (4.2, Skirk) - mentioned again in "6.0" Luna-I (Hymns of the Far North)

Related: Skirk or Childe closing the loop with Neuvillette (4.2)

[X] The Narwhal's origins (4.2) - resolved in 5.7 (Skirk's SQ)

[_] Judging the archons and Heavenly Principles? (4.2, Neuvillette voicelines: both About Us lines; Ascension: Climax; About lines for each archon)

[_] Project Stuzha (4.6, Arlecchino's SQ) - name-dropped in "6.3" Luna-IV

5.X (Natlan)

[_] Retrieving Natlan's gnosis (5.3)

[_] Capitano? (5.3)

[_] The Foul vs. Skirk? "The stars shall bow in her honor" (5.7, Skirk SQ)

"6.X" (Nod-Krai)

[_] Mistilteinn and Project Stuzha ("6.3" Luna-IV)

[_] Sandrone at the Fontaine Research Institute? ("6.3" Luna-IV)