First, Krokorok seems more like Pikachu's rival than Ash's, as has been mentioned. And I half expect Ash to catch it one of these days. Its already on pretty friendly terms with Ash and Pikachu, so it might as well join the team eventually, although it'd be weird to see Ash catching an evolved Pokemon, he hasn't done that for a while (I think?).
Although just having it as a recurring character/gag might work for a while, at least its nowhere near as annoying as some of the first recurring gags/characters. Ugh, that microphone. And its not like Ash even has to release any of his team this generation to make room for new Pokemon.
Not sure there's any need for it to end up with a rival. Its already pretty independent with how its just been following Ash, being captured by someone else who may or may not run into Ash eventually would sure slow down its eventual rematch with Pikachu.