There are several contenders for the title of GOAT, the most common being Anderson Silva, GSP, and Fedor. Every few years another flavour of the month contender will rise and fall, but every once in a while someone will emerge and truly be good enough to be seriously considered for this title. Here's what I believe the next one will require.
1. The gas tank of Nick Diaz.
Diaz shows never ending cardio in every fight, and uses it to throw ridiculous amounts of punches. He looks as ready to fight at the end of the fifth round as he does in the first. Cardio is one important piece of the puzzle many fighters lack.
2. The counter-striking of Anderson Silva.
Silva has some of the most effective counter-striking in MMA, and uses to earn many of the flash knockouts he has become so famous for.
3. The chin of Dan Henderson.
"Hendo" has never been knocked out, despite having faced the likes of Fedor, Anderson SIlva, and Rampage Jackson. He has one of the strongest chins in MMA.
4. The wrestling of GSP.
Despite the fact that GSP doesn't come from a wrestling backround, GSP has the best wrestling in MMA, and has outwrestled the likes of Koscheck, Hughes, and Fitch.
5. The submissions of Shinya Aoki.
Aoki is has some of the best jiu-jitsu in the world, and can find openings for submissions from virtually any position.
6. Decency in every other area.
You can't be the best in the world if you suck at any area of fighting.