There should definitely be a 4th.
They should have Elizabeth search for the eternal youth too.
Because she will continue to age and Will won't.
So she will become an old woman and that would be weird...
So, she should search for eternal youth and somehow fight Jack for it, they could maybe hook up, but only once - then Elizabeth will need to find a way to get on the Flying Dutchman with Will.
I think the writers should go along a story line with something like that.
Believe in Love
Believe in Keira
Join the Fight
Against the Anorexia Lies