My guess is that it would start with their kid departing for his/her own pokemon journey, following in the footsteps of his/her father, and follow him/her from there. I think they would hearken back to Ash and Misty with some regularity, and to other characters like Brock, May, Max, Dawn, etc. with less but still some frequency. My guess is that Ash and all of the main characters would end up living in the same vicinity.
How would a spin-off series of future Ash and Misty go? If they ever get married and have a kid, I'd think they'd finally settle down. I can't imagine Misty going anywhere.
I always thought that the series would end by coming full circle and showing Ash's kid leaving with a brand new pokemon, wandering into a brave new world, but a spin-off is possible too. Well, as to the fate of AAML, if the series ends how I think it will, he's got to have a kid with someone, and I think that if he's going to have a kid with someone, it's got to be Misty. I know I'm preaching to the choir here, but come on, they've done a LOT to lead us to this conclusion (or at least I think so). And as to what was said before, the dub added a few hints, but only a few. I've seen most of the shippy episodes subbed and the hints are most certainly there. Let's not forget that the dub also removed a couple of hints. A Date With Delcatty? 'Nuff said.
Of course this is only my opinion of the ending, I think its very fitting, but I'm not a writer. The ending is really up in the air and I suppose that it could very well just end without closure. That would suck.
I am a nerd.
I am proud of this. If you don't like it, that's too bad, 'cuz I'm not changing anytime soon.
Oh, and I support Pokeshipping. Years of introspection has yet to give me a full answer as to why, but I do support it, and I will support it on some level until the day I can no longer form coherent thoughts.