I agree with much of what Aduro said, but maybe not the "end of Ash's story part". There will definitely be a finality to it, but it doesn't necessarily need to be the end. If the Sun and Moon anime tells us anything, it's that despite the fans' initial knee-jerk to the slice-of-life setting, the anime prospered anyway, and I could see a future where Ash spends his life battling less, like he does in Alola now.
Personally, I could picture the season to be something very similar to XY, which contained the important milestone of Ash being a League runner-up. There was a lot of writing future writers could learn from in both DP and XY if they ever want to make a "Pokemon Master Season". Most of these writing involve Ash going through very personal stuff, being unable to control Greninja's new form and ending up in depression, having his values called out upon by a rival.
My season would tackle such personal issues, but in a more serious fashion than just a loss in a battle, or even the inability to control his Pokemon. What could be something that defines Ash's idealism that we could put to the test through conflict? Something that might very well question his identity as a trainer who loves all Pokemon equally? We would need another Paul/Shinji again, but I want someone who's the polar opposite of Ash this time, not just some misunderstood bad boy. I want Giovanni to be the central focus this time as the antagonist. It would not only give Team Rocket an important stake in walking a thin line between good and evil (forcing them to choose a side should Giovanni make them perform nasty stuff, like hurting Pokemon), but it would also reflect Ash's principles as a trainer. You could always see it in sequels that have a finality to them, an ending to a character arc. The protagonist would have some dramatic circumstance that question who they are, only to affirm that their values are the same as the hero we've known and love by the end.
I think such personal stakes should apply to Pikachu as well. Have an episode or two where Pikachu reflects on his relationship with Ash, what he means to him as a trainer. Not only could we explore the relationship between Pokemon and trainer that way, we could finally have Pikachu have some decent character development, and not just as a superficial mascot. Do one of those Pokemon-talking episodes like back in the first season so we could hear his inner thoughts, or if the writers are brilliant enough, convey his feelings with only his body movement. Maybe mix in some of Ash's old Pokemon too for this special arc. I could picture Ash's old Pokemon being asked to betray him because of something questionable Ash did (this could tie into the aforementioned Ash's personal conflict), and they then begin to ponder whether to stay with him.