Mordecai & Hassan would've been big stars? Hassan was limited in the ring, overrated on the mic, and lacked the passion for the business. Plus, all his heat was cheap heat without any staying power. That doesn't scream out "future superstar".
Mordecai was just Undertaker-light, on top of being heatless, charismaless, and bad in the ring.
And don't compare Khali to Andre. In his time, a man of Andre's size was a rarity in the sports world. Now, we've already seen Andre, Giant Gonzalez, Big Show (to name a few), plus "real sports" stars like Manute Bol, Gheorghe Muresan, Shawn Bradley, Yao Ming. Being really, really big isn't as awe inspiring as it once was. "Being big" is the only similarity between Khali & Andre.