I don't think the gameplay is getting stale. The concept has stayed the same, in the sense that you still battle with Pokemon against rival trainers, Gym Leaders, and eventually the Elite 4, but every aspect of the gameplay itself has changed drastically over the past few years. Claiming that the gameplay is stale is a bit like claiming Street Fighter IV is stale because you fight one-on-one in it, just like you do in Street Fighter II. It looks that way on the surface, but if you look closer, the argument falls apart.
There are certain mechanics that have begun to wear out their welcome, though. Specifically, the fact that HM moves have to share space with regular moves. I cannot stress how irritating it is to have to waste a spot on my team on a generic, all-purpose HM slave. What we need is a choice; when you boot up an HM, your Pokemon learns the move, but you get to choose whether or not it learns to use that move in battle. Like, Rock Climb? Strength? Cut? No. Those need to be overworld abilities only. Surf and Fly? Those are actually useful enough in combat that I could justify giving up the move slot.