What character threads would have been good movies?
Most of what works in the Star Trek movies evolve from what we know about the characters. Kirk's love for his ship and history with the Klingons, Picard's history with the Borg, Data's struggle with emotions, that sort of thing.
What character threads from any of the series could have been picked up and expanded on in movies?
I really would like to see a movie about what happened to Ro Laren between TNG's "Preemptive Strike" and PIC's "Imposters". The story she tells Picard & Riker needs fleshing out. How did she survive the death of the Maquis? How was she captured? What were the circumstances that led her to become a member of Starfleet Intelligence? How did she become Worf's handler? This all feels like it would benefit from either a mini-series or a feature-length Short Trek