While there's nothing really wrong with a Daniel Bryan/Sheamus rematch, in the current context it's a terrible idea. To be clear, I like Sheamus, I think he's a talented performer and that he could definitely have a good match with Daniel Bryan. But to have that match at this years Wrestlemania would come at the expense of something potentially far, far greater.
I've been watching wrestling for something like 25 years, and Daniel Bryan is one of the most over wrestlers I have ever seen. It's something extremely rare, and quite possibly fleeting. God knows where Daniel Bryan will be this time next year in terms of popularity, status, or health. The WWE has a chance to cash in on this now in a big way, and they may never get that chance again. And the biggest way to cash in is for Daniel Bryan to main event Wrestlemania and create that Wrestlemania moment that the entire wrestling world has been envisioning since the end of the Jan 13th Raw. And like DB or not, there's no denying that moment would be HUGE. It's all every wrestling journalist, and most fans, have been talking about ever since.
More importantly, there's a feeling among fans that if anyone else were as over as Daniel Bryan, their headlining the biggest event of the year would be a foregone conclusion. And that anything less than that has to be the result of the same much-despised backstage politics that so often creep into the on-screen product. As Madden said, if the WWE doesn't do the right thing by the fans now, when the path is so obvious, they never will. That's the perception among fans going into this thing, whether you personally agree with the reasoning or not. Anything less than the main event is a waste of potential. Is Sheamus really over enough to justify wasting that potential for his sake?
I know the WWE has lot of big names to deal with at Wrestlemania, like Cena, Orton, Batista, Undertaker, Brock Lesnar, and likely more. But neither of those guys needs to be in the main event to draw; they are already as established as they will ever be. Putting over a young, active wrestler with years left in the tank, who is right now as over as any of those stars have ever been (or currently are) at the biggest wrestling event of the year, is far more important for the future of the company than putting over part-timers who won't be around to draw fans for the rest of the year, or putting one more feather in the cap of an already established legend closer to the end of his career than the beginning (Cena). It's also more important than putting over stars that might be a big deal down the road, like Wyatt or Sheamus (yeah, I know Sheamus has been around for a while and is fairly popular, but comparatively speaking he isn't anywhere near as big a deal as DB is right now).
In short, I don't personally have a problem with DB vs Sheamus...at any other event or in any other year. But now is not the time. Whatever the WWE does with Daniel Bryan at Wrestlemania, if it's not the main event expect fans to be disappointed.