It looks that Highland can link both: Mountain and Forrest in one. I think it's a good idea because third expansions with the same board mechanics and Big Boss at the end would be boring. It is definite that one of the two left corners takes the City expansions. What would be the last board expansion then? I vote for the Wastelands (or Islands) with new board mechanics (as Timescape): That's my indications for future expansions
1. Talisman – Base game
2. The Reaper
3. The Dungeon – some thematic cards as Lesser Demon or Cave Troll has been taken from The Reaper
4. The Frostmarch – 128 cards
5. The Highland – new corner board and new way to get CoC by high Mountains - some thematic cards surely would be taken from Frostmarch
6. Dragons – new Adventure, Dungeon and Highland cards including lots of strong enemies, new climatic Spells and of course Dragon King - the Red one form base game box should appear as an Alternative Ending card.
7. The City - lots of new Purchase Desk cards and new mechanics board.
8. The Masters - new Adventure cards, strong enemies and new powerful Characters which one can became after completing some quests (e.g. in the City)
9. The Wastelands - new corner board shows that land of Talisman is surrounded by Great Sea where Strom River runs. Expansions with new mechanics in moving between Islands - some of them would be friendly others very dangerous...
10. The Tower – 128 cards – increasing the difficulties of reaching the CoC space by main board.

. Choosing something like Treasure (or characters) goes against everything that makes Talisman fun
don't have to even think about Timescape ever coming for R4th.