Well you keep the Timescape Board, more or less how it was. (Forgive me though as I have no experience with it.)
The Board represents a timestream plane; It's a transitional place both here and not here, between reality and past/present/future. You can stick around in it, but the longer you're there the more dangerous it is.
These box expansions tend to get 4-6 characters. All of them, except maybe one in this expansion, come from the future, but not necessarily the same point in it.
However in your little character blurb in the instruction pamphlet, they appear in present Talisman time. (IE the game we play all the time) Because it is the prelude, to someone finding the Crown, up to the point where someone finds it and ends that saga, which sets off the series of events to follow. (One individual with too much power, hence moving the land of Talisman into turmoil, or maybe even a peaceful state of rule, in which one or two of the characters are trying to change time for an evil goal, etc.) You have one character who is an Evil Wizard, maybe even a "Time Wizard" with a mechanic that represents manipulating time. Maybe something very powerful but at a high cost: Ditch one Spell and one Fate, take an extra turn or cancel the turn of another player.
Another Character is more or less a Golem or Sentinel (Yes, yes, Terminator Reference, I know..) that has come back in time to prevent anyone tampering with the current stream, so time can continue on to it's bleak existence. It's a Strength character with no innate Craft or Magic, but it comes armed with a Weapon card, that is just really nasty out the gate. However it's toy can be taken away if it manages to lose a combat, and at that point you have an Elf with that nasty weapon from the future running around.
The Timescape board does not need to change, nor do the mechanics or the cards; simply the ascetic details. But it is a new edition so I'm sure some of them would, to make the experience better.
As for effects that put time travel or manipulation into play on a mechanical sense. Time and Fate are often hand-in-hand in most of your fictions and mythology. The best representation of time is additional or loss of turns, but I'm not a huge fan of that mechanic, nor are many other people I know who like board games, so these extra actions would come at large costs, like discarding fate.
Maybe just the addition of a Time token, representing stolen or manipulated time.
I like simple mechanics so it works like this. If an in-game effect allows you to take a Time token, you must discard one Fate token first to take it.
The Time token is worth one extra turn of play, you can cash it in on your turn. -When you are about to cash it in, another player can cash one of their own Time tokens in, to negate the extra turn.
This would fundamentally give players the ability to buy in to the time game, or ignore it completely.
Same with the Timescape, so all those haters can play the game with it at the table, and pretend it's not even there
Haha the more I think about it, the more I wonder if I would like a Timescape Expansion even if I made it myself.
I think I would rather see a nautical travel, planar travel or just a blender expansion and keep the themes in the realm of the sane.
My opinion on this changes daily.