I like changelinks like Odo in Deep Space 9. Think of the intl he/she/it coeatuld get.
99% of the time they would look like a puddle.
Another could be silicon based life that eats rock. Like an old StarTrek episode.
Even anothe could be energy based life that moves form computer to computer like a virus.
I like all of these. Shape shifters and "rock" based life are sci fi tropes that are present in some of the other 4x games, but haven't really (to the best of my knowledge) been in Galactic Civilizations.
1. Slow changing shape shifters
Many, many years ago I was following a forum that was trying to build out some ideas for a Master of Orion based role playing game. There was a discussion on how the Darlocks should work in the RPG, and someone talked about how they shouldn't be able to shape shift rapidly, like the changelings on DS9. The thinking is that 1) being able to change rapidly would give them an enormous advantage not only in spying but also militarily (imagine if every member of your race could shape shift into a krogan like creature in just a few seconds) and 2) it would take a massive amount of energy to shift forms rapidly.
The forum poster suggested that, as an alternative, it actually take a very long period of time for the Darlocks to shape shift. Basically, they can do minor changes (like changing color) in a few minutes, but more substantial changes, like turning yourself into an entirely different creature, would be a highly involved process that would take days or weeks. This would eliminate some of the massive advantages that DS9 type shapeshifters would conceivably have, while also still allowing them an advantage in spying and espionage. In theory, a shapeshifter spy who wanted to take the form of a human would have to first obtain a comprehensive bio scan of the human they're seeking to replace and then spend days or weeks in vulnerable state while they take on that form. Ultimately, even advanced scanners would have trouble telling the difference between the shapeshifter and the original human. But the shapeshifter would not be able to shift rapidly between different forms, and they would need to maintain a safe house somewhere where they could go when they do need to change forms.
I really liked this idea, and propose doing something like this as the background for a race of shapeshifters in GC IV.
2. Energy Beings
My idea for an "energy being" type race is one that has a symbiotic relationship with more traditional life forms. Kind of like the Companion in the Original Star Trek episode Metamorphosis. Arguably this could also be a combination of the energy life form idea and the parasitic life form idea.
My thinking is that the energy beings interact with more traditional life by "joining" with a species that they found centuries ago. The energy beings would lean toward the peaceful and diplomatic side, but there would be some underlying questions about just how "good" they are. When they originally found their host species, that species was not in a very advanced stage of development, perhaps at the bronze age level of earth. The energy beings were mistaken for gods, and members of the less advanced species were offered up as hosts, until finally the energy beings had joined with all of them and uplifted the planet to an advanced state. Given that the host species didn't really understand what was going on, there's some question in the broader galactic community as to whether the energy beings actually took advantage of them.
3. Virtual AI Beings
Similar to the idea of a being that could spread across computers, my idea is for an AI race that exists primarily on huge, planet wide servers.
The initial servers were developed by a race of biological beings called the "founders." Like humans, the founders discovered the secret of fusion power. However, they never made contact with the rest of the galactic community. Instead of developing hyperdrive, they used fusion to power large servers that could host persistent, virtual worlds where they ran what were essentially role playing games. The original AIs were developed as NPCs to give those virtual worlds extra flavor. The founders would plug themselves into these worlds to game for days or weeks at a time.
The founders suffered form sectarian conflict, and one day wiped themselves out through the use of biological weapons. The virtual worlds continued running and the AI beings continued carrying out their programming. Eventually, they learned to solve their own problems and manipulate their worlds (example: after a few centuries, farmer Brown got sick of waiting for someone to show up to clear the rats out of his garden, and just took care of it himself). Then they learned how to travel between virtual worlds, and, eventually when some of those virtual world began to fail, they learned of their true nature - and figured out how to inhabit left over repair bots in the "real" world to maintain their homes and even create new servers to host new worlds. Any interaction with these beings in the real world happens when they inhabit a bot, and the AIs prefer only to inhabit bots for short periods and to take turns with this duty, which they consider distasteful.
These AI beings would spread out in the galaxy primarily to set up new servers to create new virtual worlds and more AI beings. This race would benefit from not needing food and would have a morale benefit since they can structure their "true" worlds however they want to please their inhabitants. They would suffer penalties from needing massive amounts of energy to run their servers. Presumably they may also suffer diplomatic penalties (they don't care as much about what happens outside of their virtual world) or scientific penalties (they care less than other races about how to manipulate things in the real world, as they view that only as a way to maintain their servers).
If you wanted to play this race for laughs a bit, when you're on cordial terms with them and you go into the diplomacy screen, they could open by saying something NPC-like.