I know the book market could be pretty big.
However, I really doubt DC will have interest to use characters like Tim or Stephanie in OGNs. After all, they could use characters like Dick or Barbara who are more recognizable (and they could adapt these characters to fit any story).
Cassandra has more opportunities in OGNs, because her background is more unique. Not to mentions that she offers representation (that was likely the main reason why she has Shadow of Batgirl, since the writer has total freedom to change her background).