Since both organisations seem to have an ok relationship this could work. The UFC wants the best fighters in the world and strikeforce needs depth in its roster. My first thoughts would be as follows
To the UFC: Overeem, Shields
To Strikeforce: Vera or Thiago Silva (or Both), Koscheck, Serra
Why? Trade 1
Overeem: The UFC will need Challengers for Lesnar, Overeem is a credible name to add to the division and there are a lot of heavyweights Strikeforce can pick up to make up the gap especially when Aflliction goes under(Buentello, Yvel, Rothwell, maybe Barnett) on the other hand
Vera or Thiago Silva (or Both): Strikeforce's LHW Division could use some depth and i don't see any long term direction for either fighter in the UFC (maybe Vera could work to a title shot i just don't see it), there are not a lot of LHW free agents out there and this would work out well for both parties.
Shields: this one is a no brainer, the guy wants out and has had to go to MW for challenges and he would be a competitive addition to the UFC WW division
Koscheck: Kos isn't going to get a title shot and with the fact Josh, Swick and Fitch don't want to fight each other it leaves not a lot of options, he would be worth sacrificing to get Shields and could have a really good run in Strikeforce
Serra: Since Serra doesn't want to go down to LW he isn't going to make a run in the UFC but at Strikeforce come on the build up to Diaz Serra would be golden, he could also have a good run there.
I know this is unlikely to ever happen but if it could both orgs could benefit. What do you all think? Who would you like to see move over?