It's still in the planning stages, but some people are already talking about where it should go next. I've seen some such talk in the Facebook group for it, for instance. It may seem premature, but it would nevertheless be interesting to assess the possibilities. Here goes:
San Jose - Fremont, Fremont - Oakland, Fremont - Stockton
Those possibilities were considered and rejected by the CHSRA, though the CHSRA has left them open for possible future construction. However, I've seen some transit activists sound very wounded by the CHSRA's choosing the Pacheco Pass route rather than the Altamont Pass one (SJ - Fremont - Stockton). It would make Bay Area - Sacramento service easier than the Pacheco-Pass route, because it does not detour to Gilroy and Merced, but it has various drawbacks, like cramped rights of way.
Oakland - Fairfield - Sacramento
That's a logical possibility for HSR service between the Bay Area and Sacramento, though I haven't been able to find out why that was rejected. The existing route, used by the Capitol Corridor trains, follows a rather twisty route along the shore of San Pablo Bay and the Carquinez Strait, crossing that strait from Martinez to Benicia. A HSR route might tunnel through the East Bay Hills from the Richmond area to Martinez before crossing to Benicia and Fairfield, or else it will follow the coastline before crossing from Crockett to Vallejo, turning east across the hills to Fairfield. The rest of the way from Fairfield to Sacramento should be easy, though the HSR line will likely need its own version of the Yolo Causeway.
Los Angeles - Las Vegas
There is already a plan for this, in the form of a separate system called the DesertXPress that will run between Victorville and LV along I-15 and the UP line. Its backers hope to get private funding for it, which IMO will be rather difficult. But this system could be extended from Victorville to Palmdale or to Riverside to connect with the California system. Victorville - LV is about 200 mi, and Palmdale and Riverside each add another 50 mi.
Victorville - LV construction should be fairly easy, as should Palmdale - Victorville. However, Palmdale - Riverside will require crossing Cajon Pass, which will be much more difficult.
Los Angeles - Phoenix and Tucson
LA - Phoenix is about 323 mi along I-10 or 405 along the UP line (simulated with a Google Maps road trip); the latter goes through Yuma. From Phoenix to Tucson is 116 mi, and Tucson to El Paso, TX about 315 mi. Construction should be fairly easy, though that cannot be said about a connection to Albuquerque, NM, which is separated from Phoenix and Tucson by a mountain range. Albuquerque - El Paso is 266 mi in relatively easy terrain, however. Populations: Phoenix: 5.3m, Tucson: 1.0m, El Paso: 740k, Albuquerque: 840k -- not very populous.
Sacramento - Reno
That distance is 132 mi by I-80 and 151 by the UP Donner Pass route. But one must cross the Sierra Nevada mountains, making construction of a HSR line very difficult. Furthermore, Reno has only 340k people as opposed to Las Vegas's 1.8m, and it would be hard to justify nearly 100 miles of viaducts and tunnels just to get there.
Sacramento - Redding
That distance is 161 mi by I-5 and 159 mi by the UP line over fairly flat terrain, making for easy construction, though Redding's population is 180k, rather small for a possible HSR endpoint. Chico, also on the UP route, has a somewhat larger population: 210k, but Yuba City has 170k, and the other towns there have even less.
Redding - Eugene
This is for connecting to a possible Pacific Northwest HSR line. Distance: 316 mi along I-5, 314 mi detoured to Klamath Falls, and 355 mi along the UP line, which goes through Klamath Falls. The I-5 / Siskiyou route looks more direct, but is almost continuously mountainous, while the Klamath Falls route, a.k.a. the Natron Cutoff, has some more-or-less flat area near KF and Chemult. However, even the KF route has about 60 mi of mountains near Redding and another 60 mi of mountains near Eugene.
And Eugene is not very populous, at 150k. However, Eugene and Redding are big cities compared to the most populous city on the I-5/Siskiyou route, Medford, with 76k people, not to mention the most populous city on the KF route, KF itself at 19k.
Looking beyond each end along I-5/UP, we find some sizable cities: Sacramento (2.1m) and Portland (2.2m), and beyond them, the San Francisco Bay Area (7.1m) and Seattle-Tacoma (4.0m). Sacramento-Portland would make respectable ends for a HSR line, except for the distance between them: I-5: 580 mi, KF: 579 mi, UP: 647 mi.
An extension to Las Vegas is the most plausible possibility, though it will likely need political support from Nevada.
Of the others, an extension to Phoenix and Tucson is the most likely, though it is a bit long by HSR standards, and it will need political support from Arizona. Reno, however, has 1/5 the population of Las Vegas, making it difficult to justify construction of 100 miles of mountain HSR line to it. And Redding is even less populous, though the construction would be much easier.
And a Redding - Eugene HSR line would be very unlikely.
Scaling back a bit to intermediate top speeds of 110 mph or so, Las Vegas, Phoenix, Tucson, and Redding become more likely, because of the relatively easy construction, while Reno and Eugene remain difficult. But these would not be true extensions of the CA HSR system, but separate lines that connect to it.
Site Admin Note: Merging some CA topics, and updating topic thread title. Also going to sticky.