Unlike a lot of later Fire Pro games, the method of unlocking everything in this particular installment is... pretty straightforward: just go ahead and beat the crap out of all 79 guys. It's just a matter of time before you get the Triple Crown on Easy setting - except for getting through Maurice Smith, who is one hell of a walking and breathing difficulty spike. And he's not even the last opponent in the bunch. Damn.
The video shows all three solo endings with Undertaker. Each ending shows up after you beat the HWA championship and (afterwards) either HWGP or NHC.
Also, beating the game with a pre-made wrestler unlocks a dozen of new characters plus 100 attribute points. Doing so with a CAW awards 150 points instead.