A few thoughts from a recent rewatch of the full trilogy, to smooth out the whole in the light of TROS:

TLJ’s opening crawl seems to me the best place to reference explicitly that the New Republic has no fleet now, since none shows up in that movie or TROS. (Expanded canon explains that it was mostly destroyed when berthed at Hosnian Prime, and that the remaining fleets were mainly reabsorbed by their original sovereign states out of fear.)
TFA’s destruction of Hosnian Prime has always bugged me when the Starkiller laser splits to attack multiple targets - that just doesn’t feel natural to me. I’d remove that shot from TFA edits.
Expanding on the prior two points, it’d also be nice to see an orbital fleet’s destruction (maybe with expository dialogue) during Hosnian’s destruction.
It’d take some VFX work, but since there’s a large implied time jump between TLJ and TROS, compared to TFA to TLJ being much less time, I’d prefer to keep C-3PO’s red arm throughout TLJ.
I still wonder if there’re more opportunities to seed Palpatine’s return through the first two movies. We know he’s “every voice in [Kylo’s] head”, so I wonder if his visions - and Rey’s when she’s connected to his mind - might give us a few more opportunities to hear hints at his presence.
I’d remove Rey’s line “me too- big secret” from TFA, since it implies she’s aware of her own mystery, which doesn’t really work with how that mystery box unfolds.
I also wonder if there’s any opportunity to seed anything at all about Palpatine’s hidden Sith cult or fleet. Maybe when talking about how Kylo/Snoke are force users, someone could comment “even old Sith Cults have been silent for decades”.