I'm not too worried. CDPR seem to be very careful when it comes to changing the formula - I know not everyone here agrees, but I liked most of the tweaks made to the gameplay, as well as the massive overhaul to the combat, between TW1 and 2, and the overall feel of the world and the story has largely remained intact. I don't see why they can't pull this off again with TW3, especially given that they don't have a huge corporate publisher with its hand up their collective butt like a certain other company.
Or maybe it's just that I've learned not to get too hyped about games anymore in general. As much as I love TW2, it's not a perfect game, and there are things that could've been done differently and would've worked just as well - and that's good, because it leaves room for a worthy sequel.
Long story short, every developer eventually jumps the shark and CDPR will too, but they've still got plenty of good games in them until that happens.