Would the fortunes of men in the Third Age have been drastically improved if Valandil had decided to challenge kingship over Gondor (successfully) upon Melendil's death? Afterall, he was the righful blood heir to Isildur and the 'High King' of Arnor. Perhaps the reunification of the kingdom would have been able to withstand the continuous onslaught of evil throughout the Third Age by Sauron? Of course, it goes without saying it would have to have been a welcome reunification and not a bloody civil war.
Just a thought for discussion....