Randy Savage: Regardless of the ridiculous outfits and the over-the-top promo style, Savage's talent and charisma would lead to a long stint as a main event superstar, be it as a face or a heel. He simply had the talent and the versatility to compete with virtually anyone in WWE today. His gimmick would be slightly outdated in the vein of the over-the-top 80's and 90's, but his pure wrestling ability and promo ability would help him overcome that. How would he fare with us, the IWC? Tough to say. I'm sure plenty of criticism would be leveled at him due to his look and ring gear, but would appreciate him nonetheless because of his talent. The greats have always been able to rise above gimmicks that don't fit the times, and Savage would certainly be able to do so.
Curt Hennig: I think WWE would likely introduce him using his "Perfect" vignettes, and he'ld be an immediate mid-carder upon entrance into the WWE. There was always something missing with Hennig that prevented him from rising to main event status, and I feel things would be no different today. He would be an occasional fill-in in the main event picture, but he wouldn't be a fixture there. Still, he'ld be the likely anchor of the mid-card and the guy they called upon to have great matches with top stars when needed. He wouldn't beat them often, but he'ld hold his own on a consistent basis and probably have a World Title reign or two under his belt when all is said and done. He'ld be an IWC darling that most would complain was underutilied because of his talent, although he rightfully isn't and wouldn't be a Main Event guy, which would undoubtedly irk many in the IWC.
The Steiner Brothers: In their prime, there were few better tag teams, and they would dominate the much weaker Tag Team division today. It's hard to envision them notdominating teams such as Los Matadores, The Usos and Tons of Funk, and some matches with The Real Americans could be a good bit of fun. They'ld stack up nicely with the Shield and the Rhodes as well, but they'ld likely be the standard bearers for the tag team division, which would lead to their breakup sooner rather than later. They'ld likely be spat upon by the IWC for monopolizing the division, which would probably lead to their breakup, with Scott's career taking off and Rick'd fading. Scott would be ideal as the arrogant, monster heel, or similarly as a babyface.
Raven: He's been around recently, most so in TNA, where he was well received. Today, however, I don't see a good fit for Raven, as his role and like-character is currently being filled by Bray Wyatt. He was never a top guy, and floated around from organization to orgnization, and I could see similar happening here. He would be well-received by the IWC, no doubt, but his success, if any, would squarely come in the mid-card. There's no ECW for Raven to return to, and such, no place for him to be the guy.
Vader: He's a guy I could easily see being a top heel in the business right now. Provided his ego and attitude were in check, Vader would be in or near main event programs within 6 months of his arrival. He'ld get the Umaga treatment, I imagine, running over wrestler after wrestler on his way up the food chain until an inevitable collision with a John Cena or a Randy Orton, based on his alignment. There are several dream matchups outside of such, both with big men such as Ryback, Henry or Big Show, and smaller wrestlers looking to "overcome the odds" such as a CM Punk or a Daniel Bryan. All things considered, Vader may be the best fit today(he or Savage), and likely the best received by the IWC as well.