The telephoto sensor is already 48 MP on the 17 Pro, so that's a given.
I don't think we're getting a f/1.78 lens on the telephoto anytime soon. The widest aperture the tele's ever had was a f/2 on the 12 Pro, and that was back when the tele was a 2x (52mm equivalent, so not even really tele...). Maybe f/2.4 or f/2.2 (what the ultra wide has)? That'd already be a significant improvement.
The next big upgrades on the ultrawide/tele for the 19/20 generations will likely be larger sensors - they're currently 1/2.55", compared to 1/1.28" for the wide lens. I don't think they can fit three 1/1.28" sensors in there, so plausibly 1/1.7"? But that might mean that effective aperture remains constant.
(and yet none of that will beat an equivalent focal length lens on your XT3)
The telephoto sensor is already 48 MP on the 17 Pro, so that's a given.
I don't think we're getting a f/1.78 lens on the telephoto anytime soon. The widest aperture the tele's ever had was a f/2 on the 12 Pro, and that was back when the tele was a 2x (52mm equivalent, so not even really tele...). Maybe f/2.4 or f/2.2 (what the ultra wide has)? That'd already be a significant improvement.
The next big upgrades on the ultrawide/tele for the 19/20 generations will likely be larger sensors - they're currently 1/2.55", compared to 1/1.28" for the wide lens. I don't think they can fit three 1/1.28" sensors in there, so plausibly 1/1.7"? But that might mean that effective aperture remains constant.
(and yet none of that will beat an equivalent focal length lens on your XT3)