OK, not actually incredibly startling I suspect, but I haven't seen it anywhere.
At one point in S3 Rita said that she really didn't regret having her children. She also said that she'd married because she was pregnant.
But in 3-12 we learn that she married TWICE, the first time aged 19 in 1989; the marriage lasted six months and ended in divorce.
So... what if the reason she married the first time was that she was pregnant? Was there a child? If so, what happened?
Rita had her baby in college. Neither she nor her husband was ready for the responsibility (or Rita's mother persuaded them that that was the case) and the baby was eventually adopted. Or the divorce came first, followed closely by adoption.
There is a kid somewhere, now 19 and starting to wonder about his or her real parents.
Now things get trickier - Wikipedia says that there will be a new and even deadlier serial killer in S4, and that Frank Lundby (the FBI agent played by Carradine) will be back.
Guess 2, then, is that Rita's kid will turn up while the search for the killer is at its height, and will either be the killer - not very likely in my opinion - or will be suspected of being the killer.
Guess 3 - not related to the above exactly - sooner or later Deb is going to realise that "Rudy" was Dexter's brother. She has a lot of the information she needs already, she just hasn't put it together yet. She might even guess that Dexter killed him. But I suspect that she still won't realise that Dexter was the Butcher etc.
Anyone got any objections or alternative theories?