I think that it might happen after Wrestlemania 33. I did read a report saying that John Cena might take another hiatus after Wrestlemania and as the above report says, WWE still want him to be next John Cena. So, he could seriously fill the void left by John Cena at Smackdown.PWInsider Elite is reporting that a top name being discussed for a roster switch right now is Roman Reigns. There have been discussions to solidify this move and make him the top pick to defect.
They note that not only would this get Reigns out of the cycle of competing against Kevin Owens but it would also provide an opportunity for him to compete against strong heels such as The Miz and Baron Corbin.
Other Superstars have been rumored to jump but as of now Reigns is the only person who WWE is having serious talks about switching over to SmackDown Live from RAW.
They finish by saying that the decision is focused around Reigns being the next John Cena and note the lack of strong babyfaces on Smackdown.
If it really happens, Raw might get better for some people and Smackdown might get worse for the same.
About lack of strong babyfaces, I kind of agree. Don't know if Randy or Bray are faces or what. AJ Styles is still a heel and so is The Miz and Baron Corbin. Strong babyfaces include just John Cena and Dean Ambrose. Unless, there's a full face turn for Bray Wyatt in works, strong babyfaces are required at Smackdown.
About Reigns being the next Cena, Ummmm... you can't build another Cena.