I don't think anybody from the top of the WWE hierarchy should move over to ECW any time soon. With Raw and Smackdown, you've got two 2 hour shows, so you need lots of people that can carry more intricate feuds to spread it out. With ECW, you could pretty much have just 3 matches + the normal allotted network commercial time and that's your whole show. No need for promos, no need for recaps/rewinds, etc.
Basically, ECW is a "big fish in a small pond" scenario compared to the other brands. Raw and Smackdown, associated with the two top titles in the company and being on a longer time frame, are better supported with the biggest stars out there spread out evenly. ECW can survive with the top guy on the brand being someone who's basically an upper midcarder status player on Raw or Smackdown. For instance, could you picture the WWE using The Miz as the top guy that Raw revolves around? In the next few years, definitely, but not at the moment. Right now, he's at the top region of the midcard, like MVP, Shelton, etc. But on ECW, could you picture The Miz being the champion and the focus of the show? I could, without a doubt. So by doing that, and having a wrestler that hasn't hit main event status yet be the top focus of the ECW brand, you're allowing him the opportunity to increase his credibility and be used as a lower tier main event caliber guy when he moves to Raw/Smackdown.
Plus, when it comes to ratings, I don't think it has all that much to do with moving one or two people. Obviously, Cena garners better ratings for the WWE when he's active than when he's on inactive, but I don't think Cena moving to ECW would transfer them from 1.5's into 3.2's or anything. I think ECW's three main problems with ratings are:
1. The time. Not an issue for me in the slightest bit, but some people find it tough to get through Tuesday/Wednesday/Thursday as its the middle of the week and Monday set them up to be worn out by the time they get home on Tuesday. Plus, its not an "off time" of the week, like Friday night is to most people. Tuesday night is like, bill night and stuff lol.
2. Its on Sci-Fi (soon to be SyFy...how stupid lol) and Sci-Fi doesn't get incredible ratings for anything at all. So that's saying something right there. MyNetworkTV causes Smackdown to suffer since its not available to everyone, but as far as I know (and I could be wrong) Sci-Fi is available to everyone that has cable. They just don't get a lot of traffic.
3. Recaps/Rewinds. With the addition of Superstars, we now have six hours of wrestling a week just from the WWE...on a week WITHOUT a ppv. They run those damn rewinds and recaps so much that its impossible to just sit down and watch those 6 hours on the normal broadcast. You pretty much HAVE to DVR them so you can skip forward. And if people are getting tired of seeing the same crap over and over and over again, and they're looking to sacrifice one of the shows, they're going to choose ECW or Superstars over Raw or Smackdown to miss. Even if you moved people over to ECW that they wouldn't want to miss, they'd just miss half of Raw or half of Smackdown accordingly, because they'd still see the recaps and rewinds on the other shows.
I think if you moved some big names over to ECW, all it would end up doing is prevent some people from elevating themselves up the ladder and the small boost you might get in ratings for ECW would be reflected in a small dip you'd get in Raw and Smackdown. Not worth it.