I'm a geek.
I know this is going to sound very self-serving, and I apologize for it, but if you can write comedy, you can pretty much write anything, because it's the hardest. It's the most technically demanding, the most precisely evaluated form of writing. People know if it works or not. There's a big button marked 'fail,' and that's when nobody laughs.
The trouble with a series as it gets older is it can feel like a tradition, and tradition is the enemy of suspense, and it's the enemy of comedy. It's the enemy of everything, really. So you have to shake it up.
My problem is that the audience is more fiction-literate than ever. In Shakespeare's day, you probably expected to see a play once or twice in your life; today you experience four or five different kinds of fiction every day. So staying ahead of the audience is impossible.