Even Dio wouldn't have had the necessary high range - he was essentially a baritone singer with a reasonably high range in addition, but when he sang really high/falsetto his voice wasn't forceful but mawkish. It was part of the reason why Blackmore kicked him out, he wanted a Lou Gramm type falsetto screamer for the envisaged new AOR direction of Rainbow, Dio - a great singer with other strengths - couldn't provide that.
Dickinson is a tenor - as is his (admitted) role model Ian Gillan.
Halford could have done it, but Iron Maiden and Judas Priest were actually too close for that, it would have been as if a Beatle had joined the Stones or the other way around.
Geoff Tate perhaps? He had both the drama and the range.