First, a caveat: Android 4.4 was just released for our phones a couple days ago, so you may want to wait a little while and see if any ROMs based on it come out. As of right now, there isn't a rooted version available, so 4.3 is your only option.
If you're on Bonestock, your bootloader is in fact unlocked. You can flash any of the 4.3 ROMs you find in the Android Development section. You should update your recovery first, I use TWRP version 2.6.3.0, the newest version is 2.7.1.0.
You should also flash the newer modem for the phone, VRUCML1, first, you can find that here under the Modems folder: . Only do that if you are actually going to update to a 4.3 ROM. Then you can flash any ROM you like: CleanROM 8.2 would be my pick, there's also Alliance and MOAR.