I would say the Canon R5c, but that won't be out until 2022, if at all. What I would LOVE is a full frame sensor turret upgrade for my classic Blackmagic Design Ursa. I know the unit is a beast, but I seriously like the form factor and functionality. Besides, for a camera that was released in 2014, it still holds its own. I don't know yet if I care enough about the BMPCC6K Pro enough to buy one.
I also have the BMPCC4K, along with the Ursa 4K (original), and the Ursa Mini Pro 4.6K. I like Blackmagic's stuff, but I feel cheated because they said (and still say) they wouldn't abandon the original Ursa. Aside from the V2, which only gave us slightly better performance and a FPS boost, they haven't released anything for it. I would still rock an Ursa Mini Pro12K though.