Her character as a whole and her bondings with the group is great and all, don't get me wrong there. However, we are still dealing with Pokemon here so she needs a Pokemon related goal like being a casual trainer or a coordinator or anything that would involve competitions and stuff. As of now, probably the only thing that would make the XY series longer is the mega-evolution arc and Team Flare's arc.
On a brighter side, having no goals for Serena would give her a big chance of traveling with Satoshi on the next region. Personally, I'd like to see Satoshi, Serena and Shigeru go on a journey together, whether it's in the new region, Hoenn or back in Kanto. I don't care where as long as I see these three on a journey. It's like seeing Red, Leaf and Green traveling altogether.