Hi there! I feel the same as you, new legacy challenges are hard to find. What I have done is I am being very specific in my searches - for instance I enjoy Marvel movies, so I searched up a Marvel legacy challenge and found one! It seems like there are some hidden gems if you search up your favorite franchise when you are looking up a legacy.
Try searching with keywords like character names, movie titles, and franchise names.
Hope this helps hold you over until more are created! (I am working on creating my first legacy challenge, but it wont be for a while as I need to iron out a LOT)