It has to be a combination of things they love and something that you can deal with for the duration of the time it takes to complete the project.
Does the person like a certain color or theme? What is the décor of the room in which the quilt will reside? A beautiful Baltimore Album quilt would feel out of place in a Danish Modern room. Do the person’s tastes change frequently? Maybe a heavy dose of neutrals would still work when they repaint their room electric blue a month from now.
What are they passionate about? Incorporating things that they love into a quilt is always a good idea. This could take the form of an appliqué (like butterflies) or symbolism (think of the old pattern names - Road to California for someone moving west). I’ve taken liberties with using music and freeform doodles in the quilting itself to further the quilt’s story.
What is the timeframe? The most wonderful plans sometimes have to be simplified in order to conform to harsh reality. Maybe that wonderful appliqué in your head could be scaled back and surrounded by patchwork blocks. If you have known the person for a while, think of things that excite them. A modern art lover may love a simple Mondrian-inspired block pattern in solids. Beyond that, think of their masculine / feminine tastes and outlook in general. Quilts last a long time, so don’t tie yourself down to this season’s fashion or colors. I bet all the women out there with “big hair” pictures tucked away are saying “Amen” to that one.
Also, no nice way to phrase this – do they like and respect quilts? I’ve made some that were gifts that were packed away and never used because they were “too pretty to use” and others that were worn threadbare as favorite blankies. There are plenty of threads here about quilts misused by those who just did not appreciate them. If I knew that they treated quilts like paper towels, I would make them a box of candy or cookies instead.