The Black Sabbath scenario - well, Black Sabbath fails to get ground, Iommi was instrumental in defining the sound of the band. Wether Iommi would have remained forever in Jethro Tull is debatable, maybe he would have left after some albums, but Ozzy / Geezer / Bill would have not had a similar version of Black Sabbath, with similar music and topics, with another guitarist.
The Rainbow scenario - Dio and Rainbow create more albums as good as the first three Rainbow albums, they replace the songs Blackmore would have written with Deep Purple from 1984 onwards and the Dio solo albums. They would have went on to become one of the greatest and most consistent bands of the scene, still remembered to this day. Dio would have never felt the need to join a non-existent Black Sabbath (are these situations all occouring in the same alternate universe?9.
Pretty much my sentiments.
If Iommi had stayed with Tull I'm pretty certain there would have been no Black Sabbath. If ever there was a case of the stars aligning and all 4 members being absolutely essential as many argued very well about JPJ and Led Zeppelin, Sabbath were it. I can't imagine the band having got going minus any one of the 4. As Art and Twosuitsluke posted, it's hard to imagine how metal may evolved without Sabbath. As for Tull, I'm not sure how they would have developed. Probably a couple of killer albums before Iommi worked out that he was really trying to be a round peg in a square hole so to speak.
Completely concur about Rainbow. If Dio had stayed on, it's almost inconceivable to think what might have been. Knowing the ideas floating around in both Blackmore's and Dio's heads (given what they each went on to produce over the next few years) can you imagine what a series of Martin Birch produced albums with Blackmore, Dio, Powell, Glover and Jon Lord (jumps ship from Whitesnake to reunite with Glover and Blackmore) may have been like? Like you said they would have gone to be one of the greatest hard rock acts ever.
I can't imagine Elton being a member of KC ever having worked out, but I would love to have seen what King Crimson with Bryan Ferry would have been like. Still, we would not have had Roxy Music if that had been the case. Ironic to note that KC ended up supporting RM on their 1982 Avalon tour.