CA 180 was (and, AFAIK, still is) projected as a "line on a map" west from its present terminus at CA 33 to CA 25 in the vicinity of the Panoche Road (County J1) intersection. No formal alignment has ever been adopted (the first step to actually building a facility) between 33 and 25; all iterations of "dotted line" over the years show it cutting off the present northward arc on Panoche on a more direct trajectory. The last iteration I recall (from a Caltrans map circa 1994-95) showed the route extending from the point where 180 turns NW to access Mendota due west to I-5, jogging south on I-5 for several miles before continuing west to approximately the 25/Panoche junction.
Interestingly, CA 25 north from the junction point through Hollister and on to the terminus at US 101 south of Gilroy was, during the later '70's and early-to-mid '80's, legislatively designated as CA 180 -- although CA 25 signage was retained. The Caltrans white "paddle" markers along the roadside did read "180", with mileage starting at the 101 junction (I personally observed this circa 1982-83). Obviously, the 180 extension was intended to be a principal alternate route to the Valley -- one directly serving Fresno -- to augment 152 to the north. According to my ex, who was born & raised in Fresno, many wealthy Fresnans have 2nd homes in Carmel (including her father; we used that house often) or Pacific Grove (on the Monterey Peninsula); an extended 180, combined with 156 west from Hollister, would have provided a shorter path between the two points -- and that particular group of Fresno residents would have plenty of clout vis-Ã -vis influencing Caltrans to at least engage in some preliminary planning for such a corridor (face it, no one likes slogging through Los Banos!).
The "180" portion of CA 25 was redesignated back to its original "25" number sometime in the late '80's, and the cross-Coast Range section of 180 hasn't been mentioned since. One would expect the usual suspects: lack or lessening of political interest, lack of funding, etc. But that area also poses some other issues: CA 25 essentially sits atop the San Andreas Fault for its entire length, and the area is considered "earthquake central" due to the small quakes occurring almost constantly somewhere in the area. Also, Hollister is no longer the sleepy little San Benito Valley town it once was (my ex thought it "quaint"!); massive housing tracts are everywhere that there is available land (except for some local vineyards whose owners have refused to sell!), driven by the ongoing housing requirements of the Santa Clara (Silicon) Valley to the north. 156 bypassed central Hollister with an expressway in the early '90's, the 25 bypass (in arterial form) was opened a few years ago. The chances for a high-capacity facility connecting 25 SE of Hollister to 156 or 101 are likely sunk by the deployment of all that new housing. This situation likely puts a substantial nail in the coffin of any 180 extension past I-5.