That is a good question about whether it has to be a man. To me, "actor" means one who acts - male or female. But most people use "actor" for men and "actress" for women, so I assumed it was a man. It seems that CR did, too.
I had forgotten that Matthew bought a house in Hawaii, so that does make it less likely to be him. On the other hand, he can always sell the house.
I still think it's likely that it's Dominic. I know there are rumors that he is dating Kate/Evangeline, but those are false because she's secretly in love with me. In situations like this, even if money is given as the reason, there is often more to the story - and ego has a lot to do with some of these decisions. I can see Dominic wanting Charlie to have more depth as a character and have more to do besides following Claire around and raiding the heroin plane. So Dom makes a bit a of power play and either keeps doing the same thing with better compensation, or he moves on to something else like getting back to making movies.
Now, that same scenario could apply to Sayid/Naveen. I don't know much about him, either, but he seems like kind of an arty guy who may want to use his newfound fame to get some choice movie roles and leave the island behind. So, again, he stages a power play and figures he wins regardless of how it comes out.
I don't see Locke/Terry doing this. He moved to Hawaii and seems to thoroughly love the show and the character he's playing.
Even though Josh/Sawyer may be the breakout star of the show (next to Evangeline, anyway), I don't see this being him. In every interview I've seen with the guy, he seems very down-to-earth and grateful to be on the show. I remember him saying he was planning to give up acting and get his real estate license when he got this part. I believe Josh sees Sawyer as a good role and isn't going to give it up, at least not this soon.
Not a chance it's Hurley, Jin, Michael or Walt. (The dog, maybe?)
Now, if we are including the women, I would agree that Maggie is probably the top female candidate. In addition to money and other opportunities, she may also be worried about spending too much time in a role where she is basically just a whiney little be-otch.
I have to add that I think it is a mistake for actors to leave a hit show like this. If movie producers are waving $20 million contracts at them and offering them starring roles in films, that's one thing. But that doesn't happen. I can't think of a situation where an actor left a hit show and it turned out to be a good move. I think of people like Shelley Long, David Caruso, Juliana Margolese (or whatever her name is) - it didn't exactly work out well for any of them. The kicker to me is that there's a lot of down time when you're doing these shows. You film intensely for a few months, but then you've got plenty of time to make movies or do whatever else you want. Sure, you may miss out on some good parts due to timing, but that happens even if you're doing nothing but making movies. When you've got a good thing going, milk it for all it's worth!