I can't think of any plants that would grow in the maple roots, but if it is something important I would put up a lattice fence on the ends of the deck. You would have to do away with the hand rail in that area and figure out how to support the fence. Maybe 4x4s in the ground and fasten the lattice work to it.

You have a lovely view in that direction.

Like you said, it looks shady but not dark. Plenty of light seems to get through. Maybe photinias might do well there.

Otherwise, you could grow vining plants. These could be in ground or in pots. It would also serve as an excuse that you're not trying to block out your neighbor, you're just wanting some beautiful vining and flowering plants for part of the year :-)