A Grand Highlander, a stretched Highlander with more room in the 3rd-row, makes a lot of sense to me. Toyota would finally have a true, 3-row crossover to go against the Subaru Ascent, Honda Pilot, Ford Explorer, Hyundai Palisade and Kia Telluride. (I never believed that Toyota would expect 3-row crossover buyers to buy the new Sienna minivan, despite its more aggressive styling.)
When this generation of the Highlander was introduced, I was surprised that it was neither a replacement for the 1st-generation, 2-row Venza (I believe that the 2nd-generation Venza is too small to be a direct replacement for the 1st-generation Venza), nor was it a true competitor against crossovers with larger 3rd rows, like the Subaru Ascent, Honda Pilot and Ford Explorer.
The Grand Highlander could also serve as the base for the expected 3-row Lexus TX.