A version of the 70-300 IS USM lens that isn't rubbish and doesn't cost over $1000.
And I'd like some low calorie ribeye steak, too.
It is far from out of the question. A full frame 70-300 STM has been rumoured (I think I remember a patent), and would be a logical follow up to the 24-105 STM.If it has a non rotating filter and good IQ (as recent STM lenses for EF-S), it will only lack the USM, external appearance and high price of the 70-300L.
Logic says that it will come, but only if willing to sacrifice USM for the silent but slower STM. I would have settled for that instead of stretching to the wonderful L, if the existing non-L USM had not been so dissapointing in so many ways.