The normal avatar would be able to bend all elements but the dark avatar can't here's my theory so Wan only got to bend all elements bcs of the lion turtles but now they're extinct as they might say so
so bcs unalaq couldn't get the other lion turtle to give him the other elements to bend bcs its hard to find them so yeah but if somehow the elements carried by the light avatar goes through him he might be able to bend all elements and then the cycle continues water earth fire air or just water depending if they are evil or good