My top 2 are Caspian and Elias. I love Caspian, nn Cas... the only downside I see is that it could potentially become trendy. Elias is my #2 (It was on my list until DH vetoed it... apparently it's a country bumpkin name in Iran), the only downside to this name is the popularity of Eli, Elias, and Elijah. I'm not sure which fits better, but I almost dislike the idea of giving a child a name just because it fits better with the other siblings' names... choose the name you like the best for the individual, not the one which fits best in the set. After all, once he grows up, people won't even know his siblings' names... As an individual name, I'd say Caspian... but go with your heart.
I completely agree about sibling names. I want something that's at least sort of similar in style, but I'm not terribly concerned about two names ending in "en"/"an". Thanks for your feedback!