Two metal bands come immediately to mind: Type O Negative and Celtic Frost.
Sadly, both bands are impossible to reunite, because irreplaceable, key members of both bands are dead, both bassists, both dead by heart failure.

In Type O Negative’s case, we would have to bring bassist, singer and composer Peter Steele, who died in April 2020, back from the dead. I really miss waiting for another album by them.

In Celtic Frost’s case we would have to bring back from the dead bassist, composer and sometimes vocalist Martin Erick Ain, who died in 2017. Celtic Frost was a really influencing band for the black, death, doom, gothic and symphonic metal since their inception, back in the 80s. And although we have Triptykon, with Celtic Frost’s guitarist and singer Thomas Gabriel (Warrior) Fischer, you can’t help but feel that there’s something missing.