I've said it before and I'll say it again, Allegiance is to look at as probably one of the closest games to what I:B wants to be.
It is a large team multiplayer space shooter with buildings and progression, with capital ships in the game. While not nearly on scale, it does have multiple locations to fight for. Also, while it is not newtonian, it has a relatively unconventional flight model.
It is not a perfect match, though. It has a Commander, with access to building management/construction as well as tech/infrastructure expansion, and is only in space (with rocks floating around).
While very interesting (strategy, scouts being actually important...), it is not perfect (in particular, a rather infamous learning
cliff curve), so it should be looked at for both what works and what doesn't.