Just looking at Google Maps, it looks like Santa Paula and Fillmore are adding a ton of housing, which of course is going to put more strain on a very important highway connecting 5 and 101. I know originally 126 was going to be a freeway all the way to 14, but do you think a new bypass will be viable in the future? It doesn't look like it has a lot of room to go around Fillmore at the moment however.
It looks like it would require a new freeway alignment south of existing 126, as was the case in Santa Paula. In Fillmore, it might even have to be a viaduct along the riverbed. It would certainly require taking quite a bit of farmland, but that would affect fewer people than taking hundreds of houses.
Most of 126 east of Fillmore was only two lanes until the late 90s, as you can see in GE historic imagery.
I don't recall seeing anything in the STIP about this, so it would really depend upon what Ventura and their transportation commission (VCTC) want. Ventura has never been in the big build freeways mode, so I would think unless there start to be a lot of accidents in Fillmore, it is unlikely to happen (especially with the state's stated desire to be carbon neutral). If anything, there might be a push to connect Metrolink to the Fillmore and Santa Paula area, as the tracks are already there.
I think this ship sailed a long time ago. What exists of CA 126 now is likely as good as it will get.
I've made this point a lot and I really wish they would, but I'm not holding my breath.
Considering the actually planned freeway through Santa Clarita never happened I'd say there is no chance for Fillmore.