Sure, as long as Serena doesn't interact with Ash. We know that in the last episode, Serena would want to be a better person (I know she said "woman" in Japanese but for the sake of avoiding any sexism, I'll just use "person" instead) by the time she reunites with Ash and it would be too early for her to see Ash. If she's to see Ash again in SM then it should be by the end of the anime though I still feel like that would be too early but who knows.
For me, this would be the best option. With a special episode, we'll see how well Serena is doing so far in Hoenn. This would also avoid any direct contact with Ash. In any event that Ash would need to be brought up, she'd probably keep thinking of him as inspiration to keep going or there would be some sort of indirect contact between them like a letter or video message. BTW, considering all the technology that they have, why do these people doesn't have any mobile phones which is very convenient to keep in touch with friends and family? lolz XD
IMO, this option is the least likely to happen. By tradition, new characters are introduced and if any recurring characters would reappear, it would be the characters from the last recent gen (being Gen 7). There's no room for Serena to be here unless the writers really have a solid reason (most likely to promote a new feature in the games or something like that). Serena will need a reason to be in Gen 8 and not just so she could follow Ash again. I loved how the writers handled her character in XY, there was a balance between her goal and admiration for Ash (basically, she wasn't just there just to be Ash's love interest), resulting to a good character development and that is something that needs to happen in Gen 8 if she were ever to reappear. But still, even for Gen 8, I still think it would be too early for her to reunite with Ash but if given a good reasoning then I don't see why she shouldn't.
Overall, I'm okay with all three options with the second being they best one for me. As long as the writers have a solid reason, I'm fine with how she is going to be reintroduced. Misty and Brock were able to return after all these years even for just two episodes and I don't mind waiting for Serena to return as long as the writers handle it well.
