I think that the longer it feels that XY/XYZ feels like it's in the rearview, the less chance they do anything substantive with Serena and AmourShipping. I agree with
@Manqoba that fundamentally it's just unlikely that'll they do something as big as have her get married to Ash randomly in the future without any build-up or screen time in the show.
On a more meta point about about her character and her relationship to Ash, I've always been steadfast in my belief that it was great for her character and I'm a huge fan of the double entendre meaning of what she said to Ash in her final moments. It wasn't just a confession or a statement of love for him, but it was also emblematic of her character development and the different person she became at the end of the series and that's something her relationship with Ash facilitated.
The quality of that ending is reflective of how difficult I think a return would be for Serena. Theoretically, I don't think it's hard at all and would be easy to write if the anime had a lot more rope, but I don't think it has that kind of leeway nor the level of competency behind it currently and it's those things as well as just the fundamental structure of the show that will continue to give me pause.
I still think it's possible though. The anime has been in a major state of flux and Journeys, the AU movies, and the plentiful amount of shorts are representative that. I could just be overthinking it and maybe Serena does come back and she continues to both be confident in her feelings for Ash and herself and just having those two things be readily apparent in her return would be perfect for me.