Variations upon ships, perhaps. Like the Pirates do with their frigates.
Moscows that have been modified over time to move one of their signature guns over onto the same side as the other to create monstrous broadside ships. If you still have them, could overhaul the older sprites of all your ships. Novgorods with a large gun format similar to the old Moscow, 3 guns straight down the front. A Michigan that has removed all it's front guns to overpower the built-in plasma cannon. Alien Frigates with no other guns but three of it's main guns instead of two.
I would love to see more guns added. Single-barreled versions of some larger guns. Multi-barreled batteries of smaller ones. Singular 250mm guns come to mind as a medium mount alongside the large double-barreled mount.
If, when the multi-systems are introduced, it's possible to have some fleets spawn in systems and some not, then as you get further away from a faction's capital system, their fleets get more creative to deal with life on the outer rims, thus these variations in hull design.