Well not so much a game idea but the kinds of games I like is if you play through a second time things in the world space have change.
With level streaming you can use blueprints to change world behavior so in the first play through a player would have to travel from A to B using one route and in the second play through have to go a different way. Say for example the first time around you have to go through a tunnel to get to the other side but the second time the tunnel has collapsed and you have to now go over a bridge that was gated off the first time.
Seasonal changes also comes to mind. If it’s Halloween then the map is decorated in a Halloween theme and if Christmas…well you get the idea.