I think it'll be a scaled up highlander. Not just longer, but also wider and taller. The RXL was a hot mess visually. And Lexus has a big hole in its SUV lineup where the escalade, Denali, Navigator, and also the GLS and X7. So I expect that the larger SUV will need to be more than a few inches added to the length of a highlander. The current highlander is close in size to a Land Cruiser/4Runner. I anticipate the grand highlander to be more akin to sequoia size externally. Maybe 120 inch wheelbase, 8 seats, and lots of cargo room and 30mpg, vs the sequoia being the HD version at 24mpg.

Lexus dealer want a sequoia in lexus trim, not a 3 row RX. But I think that's what they're going to get. And the LC600 is functionally a 2 row now - the rear seat is even smaller than the LX570, so currently Lexus doesn't even have a legit 3 row model. The GX IIRC now has more 3rd row room than the LX600.

I find this front clip on the highlander to look a lot better to me than the standard version: