I'm gonna stay away from the usual answers, which are most of the ones mentioned in the above posts.
Taz (1996) vs. Ken Shamrock (1997)
Back when Taz had his reputation as a shooter, a killer and Ken Shamrock was known as "The World's Most Dangerous Man". They made the shoot style gimmick famous in pro-wrestling and I would love to have seen Taz take on Shamrock when he was at the top of his game and considered the top MMA fighter in the world at the time. I would put them in a Lions Den match, it's a fitting environment for those two.
Jerry Lynn (1999) vs. Chris Jericho (2000)
Two mid-carders who were very popular with the fans and would have put on a wrestling clinic had they ever met. I'm surprised they never faced each other, they've been in WWE, WCW, ECW and all around the world, how have they never crossed paths?
John Morrison (2010) vs. Adrian Neville (2014)
This would be an incredible high-flying match. Both guys are extremely athletic and I could see these two pulling off all sorts of crazy flips, moves that make them twist & turn, parkour, cartwheels, handstands etc. This still does stand a chance of happening one day and I pray Morrison returns to the WWE one day and he steps in the ring with Neville on NXT.
The Steiner Brothers (1995) vs. The Eliminators (1995)
I've heard these two teams have actually crossed paths in ECW but it was in 6-Man Tag Team matches, never a regular tag team match. The Steiners and Eliminators are two of my favourite tag teams, for 4 big guys they could do absolutely anything in the ring and wrestle any style.
The Mega Powers (Hulk Hogan & Randy Savage) (1988) vs. The Two Man Power Trip (Stone Cold Steve Austin & Triple H) (2001)
This is one I thought of recently and should've put in my Wrestlemania fantasy card in the "Book your fantasy Wrestlemania card" thread. The Mega Powers were the ultimate super babyface tag team and The Two Man Power Trip were the ultimate super heel tag team. Miss Elizabeth and Stephanie McMahon in each team's corner also spices things up. Imagine if The Two Man Power Trip had destroyed the beloved Miss Elizabeth with steel chairs back in the day, they probably would've caused a riot, Austin and Triple H were just that damn ruthless and evil together.