Would it make sense for a feature where we flag territory? It would just be a marker for us and everyone else to see in game, but I think it could simplify border negotiations.

That's a superb idea, but you'd have to have a limited range from your borders/units that you could claim. Otherwise things would get messy really fast.

-The Kestrel

I would just see it as a way of communication between players.

A player would really be tipping his hand if he used this feature. I'd go after anyone who had flagged anything even remotely close to me.