We are missing something here. All Pokemon released by Ash in and after Johto have some similarity between themselves and that is they return back in the league except for Larvitar who was never expected to be seen again. My point is, if all Pokemon are returning then Gliscor can also return for the league and writers are hiding it from us to give us a bug surprise.
And I don't think all of Ash's reserves seen in the latest Opening will make a cameo in the league as it has happened before.
Chimchar sold more merchandise in DP than Piplup.