Interesting thread.
Kurt Angle would bring a lot of nostalgia value to the WWE and could be used as a special attraction to be used at certain times of the year like Taker & Brock Lesnar. Angle is someone who probably shouldn't do anything approaching a full time schedule in WWE if he were to return.
Sting is someone who would also bring a ton of nostalgia and initial momentum coming in. The fact that Sting is the biggest star in American wrestling of the past 30 years to have never worked for Vince McMahon is a novelty that will generate buzz. However, after the enthusiasm has died down a bit, I don't think things will be quite as rosy. Sting will always have a lot of fans and will always have his name value, but he's not what he once was and that's pretty obvious. He's well into his 50s and I don't think he could deal with the rigors of a WWE schedule. Like Angle, the best use for Sting could be to bring him in at certain times of the year to help jack up ppvs.
Jeff Hardy is someone that has a lot to offer WWE. When Hardy left WWE, he was at the peak of his professional career. His popularity legitimately rivaled John Cena and he helped generate a lot of money in merchandise sales for WWE. However, I don't think Hardy has any real desire to do anything approaching a full time WWE schedule. At the same time though, Hardy is someone whose not quite the legendary status of guys like Sting or Angle, nor is his name well known in the mainstream media as Brock Lesnar's. If Hardy had no real desire to wrestle a full time schedule, I could see him wrestling part time like Chris Jericho. However, like Jericho, don't expect any title runs or main event feuds to be in Hardy's future. While Hardy's popularity is deserving of main event attention, it's too difficult to put a part timer in such a spot. There's also issues of trust when it comes to Hardy & his past issues with drugs. WWE, I think, would be reluctant to put Hardy in major programs if they're not convinced that he's truly gotten it together.
Mr. Anderson brings nothing to WWE just as he's brought nothing to TNA. Overall, Anderson is one of the most overrated stars of the past decade. Not that he doesn't have talent, but the IWC has jacked this guy's abilities to an obscenely exaggerated level. He's not nearly as good on the mic or in the ring as some want to portray him. In WWE, there were issues regarding Anderson's health. With the WWE schedule, Anderson kept getting hurt right when the company was in the process of building him into a major star. I think he also has a Wellness Policy violation under his belt. Anderson is someone who potentially brings a lot of baggage to WWE but not much of anything else.
Matt Hardy will always be Jeff Hardy's little brother, just without the ability to connect with fans and an in-ring style that's a lot like his bothers only much more watered down. I saw Hardy on ROH tv a few weeks ago and the guy looks like shit. After having gotten himself back into shape in TNA, it looks like he's put on all the weight he lost and then some. Matt Hardy will always be something of a star because of his association with Jeff. I don't see what he has to offer WWE unless it's just someone to job out to fresher talent.
AJ Styles is someone who could bring a lot to WWE. In-ring, he's one of the best in the world. He's never been all that strong on the mic, but he's much better than he used to be. Styles is someone who definitely fits in with the workhorse wrestler like Punk, Bryan & Jericho who can be counted on to deliver quality matches. At 35 years of age, however, Styles' opportunity to go to WWE may have passed. Styles is still a young guy in the grand scheme of things but he's definitely not someone who could afford to spend a year or two down in NXT. It'd be a waste if he was to be sent there. Styles is someone that, to me, has been spinning his wheels in TNA for several years. He's either been shuffled around in TNA's mid-card limbo or engaged on one feud with Christopher Daniels after another. Styles is someone that brings a lot of reliability to WWE and that's always something any company can use.
As far as Kevin Steen goes, he's someone that brings a gritty type of guy into the WWE picture. Steen isn't really a good looking guy, he's not all that nice to look at and he's more than a little pudgy. However, he brings a kind of attitude to WWE that you don't see in a lot of characters. Steen is also someone who is deceptively athletic and has a surprising amount of technical skill. Steen is also known for being a great brawler. Steen is someone who would certainly be sent to NXT to be polished a bit and probably to drop some weight. At 28 years of age, Steen is someone that's youthful enough & experienced enough to be a potential boon to WWE.