By dictionary definition (thank you Merriam-Webster): "Firm belief in the reliability, truth, ability, or strength of someone or something."
Just by looking at that definition I can think of a few themes right off the bat: Religion, Love between couples, Love between parents and children, People coming together to make a difference, Relationship between a human and their fur baby, the list goes on. You may think not all of these ideas are good ones, and that's okay. The point of this stage in photography is to get all of the ideas on the table: Good, bad, super out-there, super cliche, doesn't matter. It's from this list that you will find your golden idea, or an idea that will lead you to it.
A way to develop more ideas is have someone write down a list of random words and try to link the word "trust" to those words with a sentence.
A great way to develop ideas is to look to other photographs of the theme you are trying to portray.