Well, I've had this idea blooming in my mind and I've written it down after awhile. It's kind of like a "Past influences the future" thing. So basically in my idea I had something like this as an example:
The party rests at a lodge and finds that one of their party members falls ill. The lodge owner says that the illness can only be treated by a now extinct herb.
The lodge owner says that the wizard who used to brew the potions hasn't left his house on the other side of the forest for ages.
Party seeks a wizard who died of old age recently. They have the idea to travel to the past and try and approach him in his earlier years.
They go to the past, and find the wizard in his former time (with obvious terrain and area changes).
The wizard speaks to you, and tells you that he cannot to anything because he lacks a certian flower which is thought to never exist in this region.
You head back to the present and let's say, you head to the "Far East" and locate that specific flower. This region wasn't even heard of in the past era.
You travel back to the past and relocate the wizard. He is baffled at how you found it, but creates it into a herb anyway.
You travel back to the present with the herb and return to the area your companion fell ill. They recover and the optional quest is complete.
This influences say things like, Bazaar goods, or the chances he offers your assistance later in the game / story or a mini quest is flagged.
That, is a crude example of it but it is a base explanation.
Now for storyline terms, I have taken this idea into further use and used it as a neccisary part of the story. Whereas you go to the past by accident due to an enemy casting a spell on you. You are seperated from your party and you find yourself in an area you reknow well. But this time it is differently coloured, the monsters associated there are vastly more powerful. You find that the terrain (as rocks and caverns, which were blocked or removed in the present,) is now drastically changed and you have more access to the area. The music is also different and the atmosphere is just totally different.
Now I'm thinking of making this a mid to low priority, and rather I see it as a "mini expansion" for quests and missions. It only appears as a essential part of the game once, and after that the player can freely explore between eras whereas they please.
Obviously, there'll be new areas, and maybe some areas are inaccesible in the past. I'll probably elaborate more later.
So, any ideas?