Extending the Romulan-Vulcan storyline from "Unification" could have been fantastic. Romulan treachery with real Romulans, real TNG-show style Warbirds, high politics and significant Alpha Quadrant consequences, a new Rommie main villain, and a role for Spock. That could have been done very well in capable hands.
Continuing from "Redemption," a good story could have been crafted with Worf, the Enterprise, and the Klingon High Council.
A story centered around Data and Lore could have worked if it was smartly written and avoided becoming campy like B4. Maybe have Lore working with a rouge group ---Cardies or Ferengis ---and draw in Data and the Enterprise.
And I might be lonely in this opinion, but I think TNG's last chapter should have been a Q film that concluded the story arc from Farpoint and AGT. With a good script and quality CGI, that could have been a great big screen Trek adventure and a proper TNG sendoff.