How about some plans involving geology? 1000 years would be on the shorter side of that scale, buy you could manage a few things.
Get a land grant at the end of a major river, and watch the delta naturally grow your land over time. Works especially well if you have a lot of swamp monsters for friends.
Negotiate a treaty for land based off of a river border. Spend some time every year re-directing the river to your advantage. It only takes a little change each season to get big gains over time.
Stake a claim to any tiny islands you can find caused by volcanoes. Watch them grow over 1000 years.
Evil characters only: Find the primary source(s) of fresh water for your enemies nations. Gradually add salt over the years. Watch your enemies starve as their fertile land stops producing food.
Also, some more uses for Decanters of Endless Water:
Do you hate someone who lives in a castle? Find a way to burrow a hole directly beneath it. Leave an open Decanter of Endless Water. Wait a couple hundred years.
Claim lands in a desert. Plant a few strategic Decanters of Endless Water. Terraform your desert home until farmland.