I've said it before, but I hope WWE does use "trading" as a storyline rather than "surprise, I am at your show now; I'm a free agent hire!" The free agent narrative is lazy and makes the "general managers" look incompetent. There is no way that Daniel Bryan, as a general manager ,would let John Cena "walk", especially given their real-life relationship that is acknowledged on TV.
The only time free agency makes sense is with NXT or new hires from outside of the company.
Smackdown trades:
John Cena, Erick Rowan, Dolph Ziggler, Aiden English, Nikki Bella
Why: Cena is already going to Raw. Rowan can reunite with Wyatt, maybe even have a tag run if needed. Ziggler needs to move but perhaps it would just mean more pointless Miz feuds, but not certain what else he can do at this point that isn't a retread unless he goes full-on heel.
Raw trades:
Samoa Joe, Karl Anderson, Luke Gallows, Goldust, Dana Brooke
Why: Samoa Joe gets to feud with the likes of AJ Styles, Owens, Nakamura, Zayn, and incoming Bobby Roode. He is automatically a top talent for the brand that can help fill the role Cena would be vacating. Anderson and Gallows reunite with Styles, but as faces. Goldust is able to interact with Breezango, which is a win to continue their skits and humor. Brooke needs a restart and would fit in on the Smackdown roster that is giving a lot of girls chances.
Smackdown "signs":
Authors of Pain, Bobby Roode
Why: AoP helps out the light Smackdown tag division ASAP. They also come in full heel and could feud with the likes of New Day and Anderson & Gallows.
Raw "signs":
Asuka, No Way Jose
Why: Asuka comes to murder. No Way Jose has nothing to do in NXT and can join the lower ranks of Raw.