Well, Expansionist ability is still very underwhelming, and I think this topic has been discussed to death, so guess what? Here's me with yet another suggestion!

- Issue: Expansionist ability is very hard to use because you both need Open Borders to use it and can't use it if the target is allied to you or your subject. This leads to the Expansionists ability being used very rarely and the overall feeling of the ability being underwhelming.

- Solution: Give the Expansionists the following ability, in addition to the current ability: "Your units can move into closed borders, but you'll generate grievances towards whatever empire whose border you trespass, your trespassing units will suffer a the "Trespasser"* penalty until they enter your territory again, and the empire whose borders you entered can attack your units inside their border without generating grievances." 

This added passive ability can help expansionists scout for more land to settle, find targets for the Annex Territory ability, and help them being more distinct than the militarists. In addition, it also creates a perfect in-game justification for the USA Expansionist affinity: it is not that they are grabbing land, it is just that they are parking military bases units everywhere.

*Trespasser =  -3 CS penalty and can't project Zone of Control outside battles 

Oh it exist? I seldom have my troops on other empires, but if they prefer that trespassing effect (health drain) to the -3CS/no ZoC debuff then I'm fine with that.