The two pieces of information we have to go by is that it is a "TOP star" and that this person wants to retire completely and already has other career paths mapped out. I find the best way to go about this is to literally dissect the roster by process of elimination so here is what I think....
There is no possible way it's Triple H so lets just get that out of the way. I don't even need to explain why it's not him.
Matt Hardy wouldn't be this big of a deal as we have known for weeks that he is having trouble with WWE management so that alone gets rid of him. This would just be another unfortunate member of the future endeavor club with no delayed announcement on WZ. Besides, I've always viewed Matt as a wrestling guy to the core so if he did leave WWE I find it hard to believe he wouldn't go to TNA.
Christian isn't really a "top" star and what else has he ever done outside of the wrestling business? I do like Christian but honestly what other offers could he possibly have?
I really doubt it's Kane. He's recently stated he has a couple years left in him and although he is big on politics I can't see any reason why a current champion would want to leave with the spotlight right on them. I also don't think WWE would keep the belt on anyone they suspect of leaving at any given moment.
The popular picks seem to be Edge (he would do movies and maybe something music related), Undertaker and Rey Mysterio (both are self-explanatory). All of these would make sense but I offer one wild (and perhaps unlikely) scenario.
Someone mentioned John Cena, maybe even as a joke but when you think about it is it really that far fetched? Who can say they predicted The Rock would leave for movies permanently when he did? Who is to say John Cena is any different? Sure he is Vince's golden boy but if he wants to make movies right now why shouldn't he? He has accomplished everything there is to accomplish in the WWE already the same way The Rock had done when he chose to leave. I am in no way suggesting these two men share similar careers because they are completely different people but they do share the Hollywood effect and I could honestly see Cena going that route sooner than we all think even if it isn't right now.
Realistically I think it will probably end up being Big Show. He has done movies in the past and has one upcoming title. He hasn't really been used all that well recently as he's being turned into some watered down jolly green giant. Most of all he's not someone that would be deeply missed by most fans and has no reason to go to TNA....well, actually, I could think of one big one: He could be the first (and probably only) man to ever hold the WWE, WCW, ECW & TNA championships, but that is for another thread.