Missing Member: Drummer Bill Ward, 2012 to 2017.
Background: Ward came and went from Black Sabbath no fewer than four separate times during their long history, but when they announced their reunion tour in 2011 he sat near his three bandmates at the press conference and said he fully intended to participate. The trouble only came when they presented him with a contract that he deemed "unsignable." This led to a nasty, public spat and Rage Against The Machine's Brad Wilk playing on their comeback album. Ozzy's solo drummer Tommy Clufetos handled the tour.
Impact on Reunion: The Sabbath die-hards would have certainly preferred to see the original quartet, but the prospect of seeing the metal pioneers one last time before they hung it up was irresistible to most and the tour sold like crazy. Sad it as may be to say, the absence of Ward probably did little to diminish sales.
Odds of a True Reunion: Quite poor. Sabbath played their final gig in early 2017 and Ozzy just announced a solo farewell tour. He's said that many times before, but now that he's almost 70 it feels more definitive. Guitarist Tony Iommi has been battling cancer in recent years and Ward had a heart scare, so it seems pretty likely that the original Sabbath will never play again.