All UC’s use the same essay prompts and they should be the same for the next application cycle but not a guarantee. The UC application opens in August. SDSU and SLO do not require or accept essays. LMU uses either their own application, the common application or the coalition application.
UC prompts: http://admission.universityofcalifornia.edu/how-to-apply/personal-questions/freshman/index.html
Common application prompts: https://www.commonapp.org/whats-appening/application-updates/2018-2019-common-application-essay-prompts
You can brainstorm some ideas but I would wait until the official essay prompts are posted.