They've already got a good premise, if a little stale considering the franchise. They just need to delve into interesting storylines and make characters interesting and engaging to the viewers. For kids they can focus on comedic elements and battles that look cool whereas for older viewers, dynamics between the groups and interesting arcs, such as depression or changing ideals can retain interest.
Like there are all sorts of interesting concepts that one could explore, like family rejection, betrayal, slavery, corruption, two sides of the same coin, common enemy against two warring sides...some of those aren't really appropriate for Pok?mon, but my point is, there are a ton of engaging story lines that can draw in older viewers.
EDIT: Also good writing of characters and plot. That's pretty important anyway but even more so in a kids show that wants to retain older viewers. Have it written well and it's a blast for whoever watches it. Written poorly and it's lambasted and only watched by the dedicated. I can't think of a better example right now than Ben 10, the original series and the remake that I watched a couple of episodes from. With the original series, there were interesting arcs that had build up and followed through to next seasons, there was a steady progression. In the remakes...they're still decent to watch, but they are definitely built towards an episodic nature and lack overarching story-lines that retain interest.
