I don't see it as a terrible idea, but an obscure side character like Nubia being more powerful than the superhero icon this corner of the DCU revolves around seems kind of...pointless.
Like is it a short cut to elevate Nubia to being more important than she actually is?
An adult Donna being able to battle Diana to a stand still is legit though. Treating the Titans as peers to the JL at this stage of their lives should be happening.