Prequel references that have been considered:

Fortress Vader (from Rogue One) added to the Mustafar minute.
Coruscant added in the background before the First Order council scene (suggested by Hal)
C-3PO mentioning Padmé or Anakin instead of Rey after having his memory restored.
C-3PO mentioning Moraband/Korriban when talking about Exegol (“the place where the Sith hid after the fall of Korriban…”)
Hint to the Sith Troopers being clones.
Rey’s power explained by a vergence in the Force (suggested for Rey Nobody editions)
Sith whispers guide Rey to the Wayfinder (done by Hal)
Leia purple saber (done)
Green mist added to Palpatine’s surroundings and green Force-draining effect to imply a connection with Nighsisters/Sith magic.
(Radical) Turn Ajan Kloss into Naboo (mock-ups done by snooker)
Familiar aliens (Twi’leks, etc.) rotoscoped in the background of some scenes as extras (the whole trilogy was lacking in this aspect).

Non-prequel references but rather Canon fixing minor edits

Cut “They fly now!” (done)
Have Zorii say Poe left her to join the New Republic, instead of the Resistance.
Not referring to Endor as the place where the Galactic Civil War ended.

Score edits: