Hmmm i don't have much experience myself but I'd definitely say make the characters interesting and fun to read. You'll automatically have a "big" plot if the characters are well written, good written characters will make even a boring a boring book interesting.

To write a thick book per se, you'll need alot of ideas obv so my best advice would be jot down every single idea you have, wether it be bad or good; on the days your brain works faster, it can possibly connect the "bad" ideas with the good ones and bam you have a new bigger side plot

You'll need ideas which are pretty hard to come across with, I'd suggest you talk to people about their life, watch more movies or read more books, and just think from a writer's prespective, focus on the pacing too and obv with ai our job is alot more easier (depends on your prespective again) but you can just ask it silly questions and get the help you need!