I'm trying to figure out which xposed modules, mods and apps are the most useful ones for the rooted fire TVs and compile a list.
What I can recommend so far:
"Xposed Additions Pro"
to customize the keys of the remote control (longpress, double press, triple press shortcuts)
No more annoying ads in youtube, twitch and other apps
Mozilla Firefox + Adblock Plus + Adobe Flash Player
Web-Browsing on Fire TV (only good with Mouse + Keyboard)
spoof what device the firetv is to actually download apps through play store (usually says that apps are not compatible on 90% of the apps, to me)
Nice AudioVisualizer of whatever audio is playing in the background (tuneinradio, mp3-player, kodi.. etc) - unfortunately a bit laggy or rather not on point with the audio.
Holo Themer (Xposed Module)
Open apps that do not seem to open on the FireTV (like SuperSu) - set the theme to "Material Dark" and they will open.
WARNING: Can cause bootloops. Use Version 3.0 as well.