Decided to try to add some sort of distance sensing and control to the Tello; it works pretty well!
The sensor are four VL53L0X sensors ($5 each on eBay) powered and controlled by a Raspberry Pi Zero W ($10).
Here's a video of the Tello moving away from my camera in four dimensions:
More information can be found here:
Contribute to aaaaronlin/tello_collision_avoidance development by creating an account on GitHub.
This is more of a proof-of-concept than an actual usable tool at this point. It would be cool to see others take this project further.