The Undertaker was absolutely terrible until about 1997.
Terrible gimmick, terrible matches, terrible feuds.
Might not be a popular thing to say, but it's true. And he got insanely boring after about 2007 too......save for his fantastic matches at Mania with Shawn Michaels and also really good ones with HHH. He became a very good worker for awhile, capable of really putting on a great show in a big match. But overall if you look at his career? Way more bad feuds, ridiculous moments related to his gimmick, and unremarkable matches. Extremely overrated in the scheme of things.
I usually hold my tongue on Taker, and this will probably get many fanboys worked up but it is how I see it. I don't go for those clown ass gimmicks.
There is truth to this but it's not as cut and dried as you are stating.
Taker was very mediocre as a worker until 1996, but that's because he wasn't given the opportunities against other opponents. He was literally fed monster of the quarter and later month until Mabel accidentally crushed his eye socket. It was literally that event that changed his path, they began to rethink how to use Undertaker and once Mankind was available it led to them unlocking the talent they didn't realise Taker had - for learning quickly from better workers.
From late 1996 onward he improved literally by leaps and bounds with each person he worked with. Foley taught him some more "hardcore" and psychology elements, Bret taught him more technical ability and helped make his work crisper and smoother, Shawn helped build the show element to it being the true "phenom" level and Kane enhanced the storytelling side and allowing him to finally work with a big man who could actually go, rather than be a burden in the ring. Make no mistake, Mabel doesn't nearly KILL him, Taker is cut by 1998 as he'd have ran his course.
Anyhow, back on topic.
In today's world it's just not possible to debut a character like that... ever.
Undertaker was arguably the last of the signings WWE made who fans would not realise were from WCW/had been high profile. Remember just months earlier "Mean Mark" was fighting for the US title on WCW PPV and as part of the high profile Skyscrapers team. He had also just filmed Suburban Commando with Hulk (who got him the gig in WWE in essence) but when he came out in WWE he was in essence a brand new man. NXT and the system today stops this from ever happening, the web would have prevented the surprise and you couldn't have a surprise today where it isn't actually an unknown coming in... big surprise debuts have to be massive names already.
Taker was also given MASSIVE rub in that first match, despite the clear botch of Brother Love as his manager. He manhandled the then Tag Team Champions, a former NWA World title and Koko B Ware... he didn't "beat them all" but it was enough to show what he was capable of and the potential. By teaming him with DiBiase it also made him seem more than "just a gimmick", although it's a shame his nadir was also involving DiBiase in 94.
There were no vignettes for him, indeed there was even a decoy with the Egg/Gobbldeygooker. That is often much maligned but the reality is the "shocking surprise debut" was Taker and the egg was the decoy... if they even want to get someone into the building secretly they have to go to airports out of town etc.
After Taker debuted pretty much everyone was known in some way or another, guys like Ric Flair, Lex Luger etc were obvious, guys like Hunter, Nash and Hall were always a major part of WCW syndicated shows, so everyone had seen them in WCW prior to their debuts... Bam Bam had been seen in the WWE prior... Giant Gonzales, Foley, Simmons, Dustin and Austin had all been major players in WCW above what Calloway had been. The last time a true repackage worked like that was Undertaker... you knew Goldust was Dustin, you knew Cactus was Mankind, Faarooq was Ron Simmons... even Kane you knew was still Isaac Yankem in disguise.
To do it today, they'd have to bypass their entire new developmental system, the performance centre and work on someone so secretively that it'd be impractical... the best they can hope for is someone like Bray Wyatt, who gets a gimmick that works, debuts and then steadily improves when given the chance to shine. They tried it with Nexus and The Shield and right now Reigns is in the exact spot Taker was in in 1991 when they were trying to make him champion... too soon... it took Taker 6 more years to get the belt again/recover fully... they don't have that time with Reigns or the like.
Even a guy like Kurt Angle was already known for the Olympics, it's nigh on impossible to think of similar case since where there was no "bedding in period", no developmental, just the debut and it was running... possibly Muhammed Hassan at the outside.
