I think it's pretty likely that The Shield will win the tag titles at some point in the near future. There's, allegedly, been talk about it happening at Extreme Rules. It seems likely but I think the title change would have more impact if they waited a bit. If THN does take on The Shield at Extreme Rules, Team Hell No coming out with a clear victory could lead to some increased heat for a rematch at the next ppv. As of right now, The Shield has beaten Kane & Bryan in every match in which they've faced off, so a big win for Kane & Bryan will put things on a more even playing field. The key would be to keep The Shield looking strong even in defeat and crank up the intensity of the feud for a follow up match at the Payback ppv. With THN getting a win, The Shield could be looking for "payback", as that's supposed to be the general theme of the show itself.
If they take the titles, I think they're going to do the Freebird Rule in which they all lay claim to being a champ and take turns wrestling in matches. It keeps the solidarity of the team in place without one of the wrestlers ultimately being left without a title. Personally, I wouldn't mind one of them taking to the singles ranks to ultimately get a singles title simply make the group a powerhouse in tag or singles matches.
No... just no. The Shield's entire shtick right now is that they are undefeated. Not to mention, they've beaten Team Hell No on two separate high stake situations, with two other stars that would technically leave them out matched. They're coming off of a WrestleMania win against Orton, Sheamus, and Big Show. They beat Cena, Sheamus, and Ryback at Elimination Chamber... These guys losing a tag title match this early AND losing their streak would ruin everything for them.
Give them the titles, let them beat Team Hell No in the rematch and then move them keep beating random opponents until SummerSlam. At that time, have them feud with the collective team of Orton, Cody Rhodes, and a returning Ted DiBiase. Legacy is still quite popular with fans, and while not Evolution, it can still work. Have a fourth and fifth member help them win, putting them at a five man team. At Night of Champions, have Seth Rollins or Ambrose win the WWE title, and then like with the tags they defend it under Freebird rules. I suggest Ohno and Leo Kruger from NXT. Ohno has an Ambrose/Rollins style while Kruger adds to Reigns' power. Then make them even more dangerous by attacking seemingly random people. First I'd have them hit Ryback, then Daniel Bryan, then Heath Slater, then Gabriel, and finally Wade Barrett. At Survivor Series, have them beat the reunited Nexus... that's right, a year of being an undefeated tag team. But stay with me.
After that I would have Shield begin to implode from sheer greatness, claiming to have nobody left to beat. And that's when you pull the Team Super Cena card. Have Cena, Orton, Sheamus, Big Show, and a "questionable ally" in CM Punk come together. Have them win at Royal Rumble but only after an imploding during the match by the Shield. From there, I'd separate the Shield slowly, similar to Evolution where they slowly lost more and more members.
But that's just one idea to go with, there are countless others.
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I don't know if this is the best way to sell people on this. Sure, Lay-Cool sharing the Women's/Diva's Championship worked, but the Diva's division and the WWE Championship scene are far from equal. The Shield as WWE Champion could similarly work, but there's a far smaller margin for error than there was for Lay-Cool. Following that thought, The Shield are still very new. There are three high-upside careers at stake, and screwing the pooch on an experimental angle could set all three back.
There's also the fact that this one would actually make a lot more sense. LayCool sharing the titles never made sense to me because Layla was the one who won the title, and she was the one who defended it most of the time. McCool was just on camera to seemingly please Taker's ego. This time, every member of the Shield would be an equal and would be a believable champion. As mentioned before, it would give a better guessing game as to which member would defend against which opponent.
Also, as it's been proven time and time again, length of time on camera means nothing when it comes to someone winning the WWE title. If it makes sense to give them the title, then give that person the title.