GM went a different route, and the SUV's came out before the new pickups this time around.
Just because the Sequoia would move to the new frame wouldn't make it the size of the Suburban. Even the 1st gen Sequoia wasn't the exact same dimensions of the 1st Gen Tundra. Toyota could've made an SUV larger than the Sequoia, but it chose not to.
I believe the Sequoia will move to the new frame/chassis of the Tundra, and get several V8 options.
Lexus just announced that they'll debut the new LX570 next month. I think Toyota focused on the Land Cruiser/LX570 combo first, as it is WAY overdue for an update. Next, I would see the Sequoia update. If not debuting late this year, then first thing next spring. Again, just all my opinion