Through my research and some DNA testing, we have determined that my husband, is a direct descendant of one of William Wallace's brothers. Very dilute, of course. Something like a 21st great grandfather. William himself had no children as far as anyone can tell. The relationship depicted in the Mel Gibson movie "Braveheart" was purely fiction. The lady involved in the story was a very small child at that time in reality.
On Family Tree DNA there is a LOT of Wallace info. They have reasonably priced DNA testing of various types as well. Good Luck!!