What system apps can be safely removed with Titanium backup on Nexus 4? Is there an official list or something? :D

I'd like to remove some of them to improve battery life, but not sure which are safe to remove
If your real purpose is to improve battery life, you just can freeze them ($6 dollars spent very well for Titanium Backup). If you don't want to spend money, i think you can use Root Uninstaller Free, that includes "disable" option in the free version: all you have to do is to open it and disable the ones you don't want.
it also includes backup and unistall options if you don't trust. (backups to sdcard/rootuninstaller i think)

you just have to pay attenction to the ones you remove, bacause no official list is findable: just don't uninstall or freeze the app if its name does not convince you like OPP, HTML visualizer, package installer, SIM toolkit,... and so on.

just do a complete backup of your device and, in the worst case, reset your phone.

everything there requires ROOT access