There are custom ROMs based on 4.3 but keep in mind that's a Google Edition firmware...and a leaked one at that. There is no Touchwiz 4.3 leak available. There will likely be a 4.3 official (non leak) of the Google Edition S4 firmware which will improve the custom ROMs.
I was thinking of rooting to stock 4.3 google edition rom. The only thing I'm worried about is camera. Will the software that helps the S4 camera still work?