I think the most obvious options you have are more robot portraits, biomechanical species, bio-energy based species, and mineral-based species.
For the former-most one, robot pops are pretty bog-standard in this game and having different portraits to distinguish robots built by different empires would be a very nice touch. Doesn't even have to be all that many robots - maybe 4 or 5 more portraits. Also, different colors between robots of the same phenotype would be appreciated.
I don't think I need to go very far on bio-mechanical. Just species that look organic but have clearly drastically modified themselves to be mechanical to a point where they are a through mix of the two, even so far as their descendants.
For the latter two, I know it says in the first post not to mention energy or rock based portraits, but I think you're lacking in imagination when you're suggesting those couldn't be done in a fitting way. Think of it in terms of energy-based species or rock-based species, creatures that might not be literally rocks (maybe reptiles or mollusks beneath their outer shell), but I can easily imagine a species of intelligent creatures evolving with the ability to form mineral shells on their bodies for protection. Same goes for energy - not necessarily energy like the unbidden. Maybe their bodies are made to easily gain electrical charge, maybe their bodies ignite easily without causing themselves harm. Maybe they're built to keep their bodies very frigged.