They should introduce a new character, a young kid, maybe a baby like Maggie. But the joke being this new character ages every year, so in a few seasons time she's the same as lisa, then the next season she's in Bart's class then a few seasons later she's being pursued by all those pubescent boys (you know - the ones with the wobbly voices) finally she goes to college, starts work and so on. I think it would be quite a cool joke. Or maybe this would work better on South Park? Any thoughts?
The nearest we have got to this idea so far was Bart writing lines on the blackboard at the start saying something like
"I should not be 21 years old by now" "I should not be 21 years old by now" "I should not be 21 years old by now"