Had Hogan not taken the role as the third member of the nWo, then Sting would have (Eric Bischoff tells in his book, Controversy Creates Cash, that he initially wanted to Sting to be the third member of the nWo, but Sting was wary to turn heel). Personally, I think this could have been just as successful, as it has always been my belief that Scott Hall and Kevin Nash were responsible for the success of the nWo, and that Hogan's only real contribution to the group was that fans were shocked over his heel turn (that is to say, had Hogan turned heel with anyone else, it is my belief that it just flat out would not have worked).
But, had Sting joined instead of Hogan, that would have left us with the same red and yellow Hogan that people were growing tired of. Furthermore, Hogan would have more than likely been the one to initially challenge the nWo, and he would have won out over them and shortened an angle/faction that was hot for about two-and-a-half years. In the end, I think things turned out for the best. But, I definitely don't think that Hogan, over the long-term, was primarily responsible for the nWo's success. I think Sting could have done just as good of a job, and, had he been the third member of the nWo, he probably would have been given that little extra nudge that would have put him on the same level of success as Hogan, Austin, and The Rock.