I don't really see the purpose of it. Most of the WCW roster, at least the wrestlers that the vast majority of today's fans remember and would want to see, have either left the business, are getting pretty long in the tooth or are currently working for TNA.
Off the top of my head, a WCW One Night Stand could only be a success if you were able to bring in guys like Bill Goldberg, Hulk Hogan, DDP, Ric Flair, Scott Hall, Kevin Nash, Sting, maybe Syxx (AKA X-Pac), Ricky Steamboat and some others. WWE could swing appearances from guys like Stone Cold Steve Austin, Steamboat, Flair, Nash & X-Pac pretty easily. Probably DDP and Vader as well. Goldberg would probably only show up if Vince paid him the moon and stars, which Vince isn't willing to do and is why Goldberg hasn't been seen in a WWE ring for almost a decade. Scott Hall's a mess, Hogan, Bischoff & Sting are working for TNA. It'd be a nice treat to see Flair & Steamboat and other guys as well but, let's face it, the VAST majority of guys that were significant stars in WWE are men who shouldn't be wrestling or even trying to wrestle due to age & generally poor physical condition. I'm sure there are a few other guys I've left out, but those're some of the more relevant ones.
ECW is a promotion that had a cult following and the cult was comprised of mostly rabid fans that ate up the product and thought of the wrestlers as living gods. I was never a huge ECW fan and feel that the VAST majority of their roster was comprised of highly overrated, talentless hacks that couldn't put on a match without chairs or tables or kendo sticks or whatever. However that small, cult status of ECW is part of what allowed One Night Stand to work.
It's an idea that sounds interesting on paper. It'd be fun to see some of the WCW guys but I wouldn't pay to watch the VAST majority of them to wrestle anymore because they just can't go. It might be a fun thing to do for an Old School Raw in which some stars of WCW, including guys back in the days in which it was still the Mid-Atlantic territory. That way, guys like the Horsemen could appear, maybe Lex Luger, Steamboat and some others. It'd have the potential to be a great, fun nostalgia show but I certainly wouldn't buy a ppv.