I was tired of seeing the US not claim Washington and northern Maine. I've put together a few simple events to fix these issues.
The first event simulates the Webster–Ashburton Treaty and gives the US northern Maine in 1842. The only requirement is that the US and UK not be at war. The other two events are for the Oregon territory dispute. As long as the US colonizes Oregon state the regular Oregon Treaty event will fire in 1846 and set the border at the 49th parallel (giving the US Washington state if it doesn't have it). If a US player has managed to colonize Oregon and eastern British Columbia the alternate 54'40 event will fire. This gives the player 2 choices, gain cores on British Columbia (and destroy US/UK relations) or accept the regular Oregon Treaty.
Now the US should end up with it's real-world borders under AI control and human players have a chance to claim Columbia.
Just put USAflavor.txt in Victoria 2/events
Edit: Tiny update, added a line to the Oregon Treaty events to make sure the US has actually colonized Oregon state as a condition for firing. I forgot to add it first time around because the US AI always colonizes Oregon state.