There has been a lot of suggestions in terms of powerups, time decay and ETC. My opinion is a mixture of all of these elements to form a living breathing world that can be both cheap and expensive for development depending on the level of complexity and scale.
However, there has been too much emphasis on going backwards. It's not just about going back, but actually going forward too. Let me explain.
Living Breathing World
Depending on the game, having a living breathing world is a key feature to consider. That's because no matter what direction you are going as a player, living and breathing worlds are always changing.
Instead of thinking of a solution to solve going backwards through a static dimension, think about how life flows in both directions. Dynamic changing events enhance both directions, which fill a game with events that can enrich a gameplay session.
This can start with simple dynamic gameplay events or triggers. That no matter the direction, provide a dynamic event for the player under certain conditions.
For example, if a player is traveling through a world zone that is outside, a trigger based on time could include elements of new music, new sound effects and a sunset that sets the mood of the zone during that particular time (i.e.: World of Warcraft).
Another example, if a player completes a certain quest, dialogs in both towns that have been visited and new towns that have to be visited include new dialog (i.e.: Skyrim).
One more example, visiting certain areas during world changing events provides a sense of player driven change such as what you expect when conquering a new universe. NPC guards now hail to a new faction, security statuses have changed and wars have begun (i.e.: Eve Online).
I focus on dynamic events because so much can be designed around them. It doesn't matter if you're doing a simple side scrolling game or a robust online game. Dynamic events breathe life into a game that is unexpected and diverse for all players that experience them. Particular focus can be applied to both new and old encounters. That means going forward and backwards can entail a set of dynamic events that really add a unique touch to the game.
I for one, enjoy change as I progress through a game. As time goes on, age and even seasons of the world make the game that much immersive. Seeing a house that was once standing during my first visit, then it burnt to the ground in my next visit is rewarding (i.e.: Fable).
Seeing the seasons change as the game time changes is also equally rewarding, even during revisits. Seeing a christmas tree erected in the main square or simply experiencing snow or fall is equally rewarding in both new and existing visits (i.e.: Guild Wars).
Therefore, I think to key to all the suggestion is a living breathing world that is built off dynamic events or triggers.