I really want to see the first generation of big whites for the RF system, I have high hopes Canon put in the effort after the assuredly poor reception of the adapted EF/built in 2X TC lenses.
300mm f/2.8 was one of my favourite lenses for a decade, though I have since moved on to 400 and 800 as preferred focal lengths. I would however love a 120-300f/2.8 TC to take along with a 400 f/2.8 TC or shove them both into one monster 100/200-400 f/2.8 TC.