You have to learn to look at things from a different perspective than you are used to. Not just with bass, but with everything. Creativity is a separate skill from any artistic pursuit, but with practice, It can be learned.
Look for and imagine what connections exist between completely unrelated things. What do Ramen noodles have to do with antidisestablishmentarianism? I have no idea, but you think about that and get back to me with a theory or two.... or else pick your own random things.
Physically explore the world and go to new places. Tree tops, mountaintops, caves. Have you ever experienced absolute darkness and silence from being alone half-a-mile underground? That sort of thing makes a permanent mark within your mind and fires up all sorts of imaginings.
Study a subject you have never even considered before, something that is decidedly not your thing. Of course any kind of study is useful, but creativity is about opening up new mental paths in the brain. Therefore, the more you feel like an alien in what you are doing, the more your brain is discovering new ways to think about stuff.
Don't believe me? Want to try something short term and more applicable to music? Pick an instrument that is very unlike bass guitar. Harmonica, Banjo, Flute, bongo drums, but nothing with strings tuned in 5ths like a bass.... so no guitar and nothing with the same notes, so no piano. Study that for a few weeks and then go back to bass. You should have new ideas.
But globally, if you want to become more mentally creative on a big scale just experience new things and think about them.