A13 and Larger Batteries
It’s basically a certainty at this point that the 2019 iPhone lineup will feature Apple’s upcoming A13 chipset (which is reportedly already being produced). While we don’t know much about the new SoC, you can safely expect a boost to performance. How much of a boost remains to be seen. The iPhone XR successor may also see a boost to 4GB of RAM.
As far as other internal component upgrades, Apple is also rumored to be bumping up the battery sizes on its new devices. That’s mostly to accommodate the new bilateral charging