Jumping the shark, is the term. It comes from an episode towards the end of Happy Days where the Fonz jumps a shark, demonstrating the desperation of writers who can’t think where to go with characters. The Will & Grace future episodes were along the same lines, though it pains us ever to say a negative syllable about W&G, you understand.
So Family Guy visits Springfield and meets the Simpsons. It looks fun but is it a bit Roseanne getting kidnapped by terrorists?,