Here's the account of how the engine and Mazda's coming front-engined rear-drive platform, dubbed "Large Architecture," will make their way to Toyota City:
The first appearance for the straight-six, predicted to come in at a hair under 3.0 liters, is the Mazda Atenza/Mazda6 successor coming around 2022. The powertrain will get a 48-volt hybrid system for increased fuel economy, and the automaker's said to be considering a plug-in hybrid version.
Toyota's first shot at the platform and the straight-six will be whatever fills the slot of the Japanese-market Mark-X sedan. We once had a version of the Mark-X in the U.S. as the Toyota Cressida. In Japan, it's sold as a rear- and all-wheel drive option to the Camry. The Mark-X is slated to end production in December this year — a "sporty four-door coupe" on Mazda's platform and with Mazda's engine eventually taking its place.
Lexus has a number of plans for the components from Hiroshima. The next Lexus IS is said to evolve from the current sedan, using a Lexus V6 but migrating to Toyota's TNGA platform. Best Car says the IS after that, perhaps sometime around 2026, will hop onto Mazda's new platform and use the inline-six engine.
Before that, the replacement for the Lexus RC in 2022 will sit on the Mazda platform and get that inline-six. What's more, Lexus will introduce a new model to slot between the $64,750 RC and the $92,950 LC employing Mazda's architecture and engine. Best Car says the model will act as a "next car" for RC owners, but we can't tell if the magazine means a two-door or a four-door coupe; the article also says the Lexus model will compete with the Audi A7.
Toyota and Mazda partnered up in 2016 on technology sharing. Best Car's take is that, as was done on the Supra, Toyota is picking up all the tech it can from suitable sources so that it can continue to sell models that don't make sense to develop alone. If the side effect of that is getting big sporty Mazdas to go with it, well, so much the better.