Nostalgia refers to the sweet memories which an individual would want to relive all over again. The Nostalgic concept in Paradise lost is based around the lives of Adam and Eve. The moments that they spent in the Garden of Eden were cherished by both Adam and Eve and those moments later turned into nostalgic memories in their Lives, this happened simply because they were tempted by Lucifer and ate the forbidden fruit. Eve was the first to eat the forbidden fruit and Adam committed a bigger sin by following in the footsteps of Eve. Adam and Eve start having differences after the consumption of the forbidden fruit. This is exactly why the title is "Paradise Lost", they do not realize the simple fact that they have lost the Paradise which they were in by quarreling over small issues. When they quarreled over small things they realized that things had changed after the consumption of the forbidden fruit. God decides to expel Adam and Eve from the Garden of Eden. Adam then goes on an expedition with an angel to know how the world is like. He finds suffering in the world and becomes nostalgic to the moments he spent with Eve in the Garden of Eden.
An Ideal world would mean a place where there is no suffering and there are a lot of good things to look forward to.