#2. Daniel Bryan

We're surprised to see that this match never came about between 2011-2014 at any point. While the two did have a brief alliance when Daniel Bryan was with Kane as a part of Team Hell No, the two veterans never had a Singles match against one another.
Then again, the two have been babyfaces for the entire time, with Daniel Bryan only having a heel run in 2011-12 and for 10 months from late 2018-2019. There wasn't anything that specifically warranted a match between the two, but had Bryan stayed healthy between 2014-15, it wouldn't be hard to imagine a blockbuster Big 4 PPV match between the two.
With The Undertaker's change in character, we could see a feud with great promos between the two and a great match, since Daniel Bryan would be doing most of the heavy lifting. However, the two have always been on different paths and it doesn't seem like there's much of a reason for WWE to book a match between the two now.