Nobody said that it's needs to be a normal wrestling match, but it also doesn't need to be a brawl, that's wrestling, you're not working a years of long term booking unless it's the same guys, now with HHH out of the picture, you got a different opponent, an opponent that can be better than HHH, more effective than him with a bigger chance to beat and match Taker's style, I'm not talking about Rock or Brock or any specific person. Just the context, from what you said, HHH and HBK are the best of all time and because Taker beat them in that type of match, nobody else can do it. Let's take this for example, because we're mostly talking in the borders of kayfabe land here, Chael Sonnen took arguably the GOAT in MMA Anderson Silva to the limit like nobody did before him, just killing him for 4 and half rounds, in a desperate move Silva was able to win the fight via Submission, now if another fighter, with different style and better skills, say Jon Jones, goes in and beat Silva in 2 rounds, does that mean that Silva was unbeatable because he fought one person for 4 rounds? no. Just means that he fought different styles and different caliber of opponent. Same thing with wrestling if you want the story to be fresh and exciting and not the same brawl every year with the predictable nearfalls and kickouts, nobody is more credible than The Rock today, a guy who beat Cena clean after 8 years. Don't get me wrong, I can get your view on this subject and personally I would much rather watch Rock/Brock than Rock/Taker, but that doesn't mean Taker suddenly became a repetitive one style guy. You're not giving these workers enough credit for their tremendous mind and ability to lay out a match IMO.
It's not about the opponents/previous opponents being the GOAT or anything even remotely close to that. It's about how much further things have to be taken every year in order to get Taker to stay down. It's character based. It's all character based because the character of the opponent is linked to their credibility. HBK is a performer/wrestler etc etc. He tried to wrestle Taker twice and failed. HHH is a fighter/brawler. He tried it his way and failed twice. Both HHH and HBK are 2 of the best in their respective fields not to mention their legend and all their accolades. If Shawn Michaels can't beat The Undertaker at Wrestlemania in a wrestling match, The Rock/Cena/Brock sure as hell won't be able to do it. If Triple H can't beat The Undertaker at Wrestlemania in a brutal fight, well, he may have failed but to your point, that leaves the door open for somebody else to come along and try their hand at being even more brutal than HHH was willing to be in order to get the job done. Enter Brock Lesnar. The common thread between the HHH/HBK/Taker story arc was respect. The closing image on them as brothers in arms is a powerful one. Brock Lesnar doesn't have that same respect. HHH is ruthless but even he showed signs of mercy in both matches. Brock Lesnar won't. He's he perfect opponent.
It's also not about Taker or anybody else becoming a one style guy. The evolution of the streak matches is all you have to look at. I don't see how you can say that reverting back to a less brutal type of match makes sense tbh. It doesn't. Rock beating Cena gives him credibility, that along with the fact that he's The Rock. But streak matches aren't just any matches. You have to have that element of believability that the opponent has the tools to end the streak and can then go on and end it. It would be fine for them to continue the less brutal type of match had 27 and 28 not gone down the way they did. But after those matches the message is clear. You must literally KILL the Undertaker to beat him at Wrestlemania. Not outwrestle him. Not outsmart him. Not out-entertain him. Not any of that because he simply will not die unless somebody ends up pulling the trigger for him. That's the story of the streak matches now. To change that would be to undo all the tremendous work of the last 4 years and to put quite the dent in one of the best story arcs WWE has probably ever constructed. For that reason, Brock/Taker is the way to go with Cena/Taker at Mania 30. Rock isn't even a part of the equation. Unless he's willing to change from his current legend/icon/entertainer persona into a vicious merciless fighter, something that he's never been btw, then it won't and can't work.