Heathermom2opmc, The fabric you chose is up to you and what you have on hand. I know that some swappers shop for their swap squares or at lease some fabric, but it is not expected. Some follow a theme, others do not. I will let you know what I have done. Last year I sent all batiks, but that is what I had on hand. Other years I have have followed a single color way (all orange or all purple) or rainbow colors or red/white/blue. For several years I chose a song theme, or poem theme. But that was just me. You are absolutely free do as you choose as long as you send 50 different squares. I have gotten swap envelopes that are random squares and themed which are equally useful in the quilts I make.
While here, some folks send small special surprises or trinkets in their envelopes. Again, it is not expected. In the past I have gotten a fussy cut square that said Happy Birthday, a little birthday bookmark or little hearts or a pattern that makes use of 2.5's. Please, please, please don't feel obligated to go beyond the 50 squares.
Thanks for the good question. I am sure that other will chime in too with there own experience.