Our "killer app" thus far, in terms of useful things that actually require root, is PicCap [1] - which enables low-latency framebuffer capture for DIY Ambilight systems.
In the just-for-fun department, you can replace the default screensaver animation with a bouncing DVD logo. [2]
Some telemetry is blocked unconditionally during startup, although it is incomplete [1] (Perhaps I was a bit over-optimistic when I said a pihole was not required).
Custom CA certs are possible, which people on older models have been using to work around issues stemming from the X3 cert expiry [2].
Nice, I’d love to install software such as rpiplay [0] to mirror my iOS device, also would be nice to maybe gain some kind of low-latency wireless Linux-compatible screen share server.
It's one of the (few) apps on the Homebrew repository that gets installed (the repo, not the app) by default after you root: https://repo.webosbrew.org/apps/
I tested it briefly yesterday evening, seems to work fine.
In the just-for-fun department, you can replace the default screensaver animation with a bouncing DVD logo. [2]
You can see a list of currently available homebrew apps here: https://repo.webosbrew.org/apps/
You can also block telemetry, updates, etc. (without relying on something like a pihole).
[1] https://github.com/TBSniller/piccap
[2] https://github.com/webosbrew/custom-screensaver