According to an article in Automotive News, Toyota plans to move the Tundra and Tacoma to a shared platform, internally called "F1". The platform is to be used globally and will require a bit of modularity. The platform is said to be nearing completion and the next Tundra could be debuting for the 2021 model year.
For US consumption, all Tundras and most Tacomas are built at the company's San Antonio, Texas plant. Moving to a single platform would make things easier and cheaper on Toyota. But what of the other SUVs built on the same platforms? The Sequoia is largely based on the Tundra and the 4Runner is related (distantly) to the Tacoma. Those vehicles would likely be refreshed right behind the trucks. In any case, Toyota is planning on putting some electrification into the new platform.