A small piece of the musical theme that plays during the buildup to Yoda and Palpatine’s duel reappears during Rey’s Ajan Kloss training (granted, that piece of music goes back to a very similar-sounding motif in Empire Strikes Back).
The pirate Sidon Ithano at Maz’s castle (the one who Finn approaches to run away) wears a Kaleesh mask. General Grievous (a former Kaleesh) wore one such mask as a faceplate.
The Jedi being mentioned in dialogue as an Order.
The idea that the Jedi fell because of their own incompetence, as portrayed in the prequels and fleshed out in Clone Wars.
The ships among the citizen fleet in TROS include Naboo N-1 Starfighters (those are the ones TestingOutTheTest refers to. The Nubians are the royal cruisers)
The existence of only two wayfinders might have been a reference to the Rule of Two.
The helmets of the Sith Troopers greatly resemble the Phase II clone trooper helmets.
The “Do it!” Palpatine line in TROS can’t be a coincidence, can it?