I think the map will be ok if we change the victory conditions to something like
80% of the central isle, with holding a castle counting as possession.
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No good deed goes unpunished...
I like the idea of the fighting to the Last team. No need for an 80% land victory because if one team gets that much the other teams may concede the game to that team or may want to fight it out to the end. I always find the fighting to the end part the most fun.
Tuidjy did not say anything about controlling 80% of the total provinces, but merely 80% of the provinces in the central isle (that would be a fair number of provinces, though it could be achieved if opposing teams were playing a bit too defensively).
One potential worry about using Theater of War for your team game: Only one team at a time will have control of the two provinces needed for travel between the starting area and the central circle. That could make fighting a war a real logistical nightmare.
Those two provinces each have preset fortifications there. One nation can own both forts and the other one can travel through it freely. Basically leave 0 pd in both provinces and its fine. I like the one province chokepoint idea because it is different than normal games. It makes things interesting.