I have been saying this for weeks... It's a far better "Dual Main Event" when paired with Brock/Rock than Brock v Reigns will be.
Yes there is some "hot potatoing" involved to make it work... Ambrose has to beat Brock somehow, only to be cashed in on and then Reigns takes the belt at mania... but the positives would outweigh the negatives here... You have 3 guys who have been brought along at the same pace, and are all shining since going solo... Brock v Reigns is a short term view in putting all those eggs in one basket... allowing the 3 to fight for the belt while Rock and Brock help shoulder the "buys" of the event means that you have 3 bona fide main event stars created out of the situation... Brock can easily be first opponent to Reigns or Rollins or Ambrose could then get their proper turn over the following year depending how they go...
Make no mistake, right now is a TERRIBLE time for Ambrose to be off TV as he is the most over... a shock return/win over brock at TLC would be a brilliant kickstart to the "new Austin".
Remember Rock Austin and Trips all got their first titles very close together...so why should the Shield not do so either?
In regard to your last comment...Austin had his first title run last 91 days. In this scenario how long does Ambrose have it for? 60? That's at best. Stone Cold lost it for a day to win it back for another 90 days. So there's at least a 120 day difference to your Shield comment.
Then the Rock won it and held it for 44 days, which is about 10-15 days different to what you're suggesting.
Then 277 days later Trips wins his first world title. So you're suggesting TLC-Wrestlemania is a viable comparison to 390 days that your 3 guys took. It's completely different. You wanna give all 3 of them a title opportunity in the length of Stone Cold's first 2 reigns. And that's getting them over?
Yes I'm going to the team argument. The Shield broke up in June, so by Wrestlemania they'll be singles stars for 10 months lets say? Stone Cold made his WWF debut in late 1995. He didn't win it till March 1998. So he had 2 and a half years as a singles wrestler before he won it. How about the Rock? He debuted Survivor Series 1996. Winning his first world title, at Survivor Series 1998. So you have 2 years. Less time but still. Now Trips debuted in April of 1995, it took him till August of 1999. So over 4 years. You can argue he was with DX...but that was for 2 years

even during the team he fought for mid card titles.
So this is nothing like that, I'm ignoring everybody's WCW/ECW past, so it's only fair to ignore any indie experience Ambrose and Rollins have.
All I'm saying is that this plan would hurt more than help. Brock versus Rock could be a draw, I'm not arguing that. Hell, I'd love to see a triple threat match between the Shield guys but not for the title. Maybe a mid card belt if it's needed but I don't think it is. Ambrose returns maybe in a month, you'd have to keep the feud fresh for 6 months through 3 gimmick ppv's.
Here's what I would do, run with the current plan of Reigns v Lesnar. And in a year or two set it up slowly. No title on the line, just who was the best member of the Shield. Of course, Reigns will win but it's an idea.