As of right now, the way it's intended to work is that you start the game in a relatively safe system, and the farther you go away from that system, the more dangerous the universe becomes - not by design, but simply because it generated the universe and then picked a safe system for you to start in. When you move outside the initially generated area, it creates more systems, more factions, etc. These factions can become more powerful in exactly the same way you can, and of course, they'll want to expand - and you'll be in the way. While some AI personalities will be content to coexist, with others it will lead to large-scale wars.
In addition to the regular "blow sh!t up" approach, you could also target the enemy's economy if you're careful and skilled, bringing them to their knees despite being a much smaller force... provided they don't do the same to you. You could alternatively persuade another faction to go to war with them, weakening them for you... you get the idea - multiple ways to accomplish any major goal.
I'm not sure if that's the "direction" you're looking for, but I guess it's a start? It's not quite sieges, but espionage and war could keep it interesting for a while. If you don't want to go that route, there's also exploration... Josh has made exploration a perfectly viable path through "data packets": basically, discovering new things - like a wormhole - generates a data packet with the location of said wormhole, which can then be sold for a profit - larger depending on how many other people know of it.