File this in the drawer labeled "Only Remotely," but Mazda U.S. VP of product development Robert Davis allowed that the company is considering a diesel engine for the next Mazdaspeed3. Based on the report in Car and Driver, it looks like that's as far as this has gotten: a diesel is on the table. Beyond that, none of the 'pros' mentioned seem to make a case for it, and the 'cons' make a case against it.
The idea is made possible by the fact that whippersnappers getting into a Mazdaspeed3 aren't against diesel engines, being more familiar with modern clean diesels than the particulate monsters of yore. But if Mazda boosted its diesel Sky engine for the purpose of being a high performance engine, the frugality benefits of the engine would be erased. Then there's that whole issue of additional heaps of diesel torque, and the additional heaps of torque steer that Mazda would have to engineer out of the car.
So it is perhaps best to take from this that a diesel engine for the next Mazdaspeed3 is a distant option, which could be seen as another crack in the wall. And while "Only Remotely" doesn't mean impossible, we wouldn't place any wagers on such a thing happening this next go-round.