Decoding rotary encoders is a common task, and very hard to do with Microcontrollers at either high event rates or multiple instances.

Maybe even turning a brushless motor with an encoder into a servo motor.

My main idea for using FPGAs would be talking to a couple of i2c/spi sensors and reducing the data rates. Some sensors only have one or two possible i2c addresses, some require multiple requests to get a limited amount of data, reducing the polling frequency.