There is one.
Now, when I say that I mean not only in complexity, but in theme. A true 4x space conquest galore. It is Space Empires, in two flavors: Space Empires V with Kwok's balance mod, or Space empires IV with any of its great mods. There is simply not another game that comes closer to Aurora in terms of depth and complexity, even if Aurora is still more complex. As stated in the first post, perhaps the only other reasonable comparison would be Dwarf Fortress.
I wish someone could prove me wrong showing me another complex great Space 4x. . .
I hate you. Really I do. Why did you have to remember me of space empires 5 with the kwok mod? Now I HAVE to reinstall it....
War in the pacific AE is immensely complex, and immensely long. Like, LONG. One game might take 2-000 hours or more. There is a catch though, if you really want to play it, you have to play it against a human opponent via play by email. On the Matrix forums there is an entire forum full of epic AAR. It's THE ultimate strategy game. But as said, it's frakking LONG. And it's learning curve, if you truly want to LEARN it and play it well (and you will need to do so against a human opponent) is so bad that Aurora is probably easier. ...
Distant worlds is extremely nice, but requires you to auto pause very frequently if you want to micromanage everything. And people who play Aurora will want to
Dominions 4 is quite complex. But very similar to Dominions 3. And while the game is very good, I didn't like too much the endgame specifically.
About AI war, it's a completely different type of game, BUT I have to admit that depending on the setting you choose at game start it can be insanely difficult. As in, why was I so masochistic as to try this game like this?