IIRC, the Supra project has been shelved indefinitely by Toyota. True Supra enthusiasts/purists say the Supra is dead as of the MKIV and anything else coming out is just carrying the name will always lack true Supra DNA.
If there is a sportscar shared with the IS platform, I think Toyota will likely badge it as a IS-coupe and call it a day. But in any case, I don't think Toyota has any real motivation to bring back a sportscar. The closest to that we can get to a sportscar today is the X-Runner.
Toyota is more an appliance maker. They just build plain vanilla cars, but they do a pretty good job at it.