This thread is speculative and hopefully just fun, for those of us who wonder what happens when all corner expansions have been published and seemingly there may not be new territory to consider.
We can consider how future small expansions could create new ways of advancing play without adding any/many cards to the main Adventure Deck.
For example, a small set with it's own deck and theme that is related to Fate/Dark Fate. Re-define or enhance the use of Fate tokens to consider the powers of Order and Chaos - not the Evil-Good-Neutral continuum. When we pay Fate to re-roll for movement or an attack roll, we're kind of saying "I know I could have swung better, or traveled a little further/or just to where I wanted to". But when we pay Fate to re-roll for the Witch or the Enchantress, that seems more random, "I'm hoping the Gods(?) don't let me become a Toad/Lose all my Fate, etc."
With this basic idea, the use of either dark or light Fate can be scored, to create a threshold effect that is either good or bad, random or destined, etc.
You could add the ideas of Pantheons or God/Demon like beings who thrive on Order or Chaos, and a specific deck for these powers.
In short this would be a small deck expansion - without a board overlay or addition - more like the Dragons. In keeping with all expansions, it would not depend on owning any of the others. So any while it may enhance the Night/Day, Dragon, Warlock's Quest/Rewards, Reaper, in some way, it would not reference them.
but what about a final expansion that is considered "Master" class or Advanced Talisman?
Well I would like to see such a game and with certain requirements or considerations.
Special portals or gateways that could teleport between the regions, and possibly the other Runes/Runegates. Six of these, 1 per Region, except the Inner/Dragon Realm. So if you are playing with less corners, you omit 1 of these gates for each.
At the beginning players take turns randomly placing the gates, 1 per Region only, save the Inner. When a player lands on the space with the gate, they encounter the space as usual. They can only encounter the gate on the beginning of their next turn. At that time they must pay 1 Fate (possibly to scored as either Dark or Light) and can teleport to any of the other similar gates in the various Regions, or they can also teleport to any other Runegate, or runes space. (But this cannot be done in reverse; you cannot use Rune/gate to teleport to one of these - because there is no payment to Fate for those)
I feel that facilitating this 'travel' would be a good feature for faster game movement in general.
Another way to create connections between expansions and the final game could be the use of some new NPCs, different than the Reaper or the Werewolf. We have certain Followers who have semi-npc characteristics. The Prince and Princess, notably. What if there were a King and a Queen with other agendas or concerns. Royal kidnappings, imprisionments, creating reasons to journey to the various realms to unite these characters or deliver back to the Castle. Or tied into rewards that give advantages (Keys to the Kingdom idea I had before?).
Other ideas for expansions, either 'end-game' or not could include: Demonic/Godly realms (more overlays, or board-lets between corners?), A Non-player Character, maybe that Old Wizard, someone that doesn't have the base goal as their goal. What possible character could this be? Besides the Old Wizard, who is possibly a Lich now, characters that are Zombie/Lich/Vampire related and whose goal is to turn the entire realm into a land of Death. How could this be accomplished vs the normal conditions. I have my own ideas, but this is pretty crazy. If the Wizard/Lich gets to the Crown of Command first, he can open the Black Void and swallow the whole game, select random Dragons, Spells, adventure cards, etc. to attack other players.