I actually think the writers are in a very tough position considering the development of Pokemon. The franchise has been basically running for a decade with no real plot development and thus is getting very very stall. Basically every single series is the same adventure. Right now the writers realize that a relationship involving Ash would be really great for his character development and add a whole new dimension to the plot, however by doing so, the plot will be basically set-up for a perfect ideal ending which is probably not the intention of the writers as they want to keep the franchise running. It is a dilemma for them because if they continue with the tradition I am sure Pokemon will fall into obscurity. If Ash x Serena does happen, the story calls for an ending.