Well Misty wasn't relevant to AG and May wasn't relevant to DP, but the main reason they got cameo's was because they weren't that far removed from those saga's that the writers couldn't simply make them relevant again.
Not really.Gary was far removed by not being brought back in whole Hoenn and Battle Frontier with writers making him relevant again.
Jessibelle was completely irrelevant with writers making her relevant to James story again,Butch and Cassidy had no reason to return with writers making one becoming relevant to current DP story.
How much character is absent isnt dictating his chances of being brought back.
A Paul cameo in BW wouldn't be that big of a problem because they just left Sinnoh so it wouldn't be hard for them to make up a reason for his return, but a Misty or May cameo would be harder to execute because of their relevance pertaining to current day Ash.
Again bringing up excuse behind Misty,May return wouldnt be any harder than it would be for Dawn or Pauls.Thats the smallest problem in here.
The only reason why Dawn for example has bigger chance to return is because she was recent main character still not making usual mid region cameo like girls before her did.
Paul didn't appear nearly as much as Dawn, but without him DP would have been like AG all over again. Paul effected the story of DP just as much as Dawn did, so I think they're on equal footing in that respect.
Generally speaking Dawn was overall more important than Paul was.She received much more screentime,her contests influenced Ash storyline starting to use techniques from coordinators in his battles,her contest arc was given much more attention and focus affecting greatly plot of Sinnoh,so no they cant be equal in any way or form no matter at what angle you look at it.
Of course they always have the closing word, but it's the reasoning behind their decision to bring back a character that needs to make sense. If they just bring an important character from a past saga back for no reason and they have no relevance with the current characters or plot, it doesn't make much sense to bring them back.
And your forgetting how writers if want can provide reasoning behind someones return.There are plenty of valid justified reasons these characters could make appearance or even return to main cast.
Reason why some of them didnt returned is more because of too many older characters being present already with there not being room to show all of them in every region than anything else.
Well just because something is entertaining doesn't make it of good quality,
Im sorry but your wrong,if character fills criteria of being enjoyable to audience liking his aspects and storyline he already made success making him good quality character.
Your mixing quality of writing being present in someones story with quality of character itself.
But the bottom line is, Paul was a better rival and character overall than Gary, whether Gary or Paul was more entertaining is completely a matter of opinion, but that doesn't change the fact that Paul was handled better in every aspect. Entertainment value doesn't always dictate the quality of something, and in this case, it doesn't.
Again matter of opinion.Paul had better written storyline but claiming how he was "better" rival and character to Ash is subjective with final judgment being on viewer itself.
For example you might claime how Paul had better battles,better interaction with Ash etc but on other hand something you find being good other side might view differently with Gary in his view being better battler having better chemistry with Ash.
You can argue about this all you want,but whether someone is better or not depends on viewers personal interpretation with lenght of development not determing this.