With all of the outpost spamming and pioneers sneaking around in your territory, likely you have some jerk of a pioneer plop down an outpost in some inconvenient place like a road or a choke point.  Your only option to get that territory from him is to go to war, which in this game tends to last until someone is wiped out.

It would be nice if foreign outposts (and perhaps cities) had something like a "Buy" button on them that put you into trade negotiations with the other player for control of the outpost.  The value of the outpost would depend on the value of the resources and improvements within its ZoC to the owner.  If you could also sell your own Outposts, you could almost barter territory and consolidate your holdings, resulting in more defined borders instead of the patchwork of locations were players have managed to grab territory.  I don't know how you would do the interface, but being able to cede or demand territory for peace would be an interesting diplomatic option.