Maybe do it like in civ5, nobody can enter inside your borders unless you are at war or sign a treaty of nonaggression / free passage.
But I think some changes should be made regarding outposts if that is implemented. Is very easy to "secure" resources or locations especially for Pariden with magic outpost spam and deny other factions access until research for agreements is done and accepted by both parties or DoWs.
It's something one must carefully threat to not break the strategic aspect and exploration of game.
Say I'm in an isolated spot at start of game and another AI in vicinity who completely blocks me access early on with his borders from entering mainland to find expansions places and resources. What do I do? Sit with 1 town in my corner with no resources (crystal and metal for example) trying to research diplomatic relations in the hope he will stay friendly toward me (most unlikely since he will expand and gain more faction power, especially if we're different beliefs, kingdom vs empire) and let me pass through his territory, or will I try to go to war early on with no economy or production power and no access to upgraded units?
CTRL+N is not the answer for this, a set of rules / balance must be set to not completely break the game