Okay. Here is my idea for an original race. It is a "body snatchers" trope and pretty similar to another sci-fi race in existence.
Tegrum
The Tegrum are a species of small, sentient organisms, dependent on obtaining a symbiotic bond with another organism for survival. In exchange for the nutrients and protective outer organism of the host, the Tegrum secrete a mixture of chemicals that improve the physique and cognition of the host. (Not really sure what a Tegrum should look like outside the host.)
Origin
The Tegrum originated in a distant galaxy, long ago developing a drive to "unite" all races they encountered. Their diminutive size often meant that even should a host be destroyed, they could survive until recovered. Though a Tegrum will usually do all it can to protect its host, especially when a replacement may be difficult to come by. Mature Tegrum are capable of gaining full control over their host for periods of time.
Those Tegrum that journeyed to the Milky Way all had the same host species that would also come to be known as Tegrum. The hosts are humanoid, though lacking in defining features, more resembling "blanks"(simple clones) than individuals. Though no larger than a typical human, they are heartier with prolonged lifespans and the combined intelligence of host and symbiont.
Stages of Symbiosis
The Tegrum have a complex life-cycle, largely dependent on inhabiting a compatible host species. Traditionally, the Tegrum seek volunteers to become hosts, believing that only the willing can benefit fully from the mutual existence. However, do to the heavy opposition of the natives of the Milky Way, some Tegrum have begun to force the symbiotic relationship upon hosts in order to ensure the survival of their offspring.
Stage 1: Larvae: Extremely young Tegrum are incapable of survival on their own. They also cannot survive inside a host, lacking the development to suppress a host's immune system and integrate into the body systems. They are kept in large "ponds" aboard their progenitors' vessels, where caretakers tend to them, ensuring optimal growth and development. These "ponds" function similar to a brain and caretakers can input data to teach the Larvae.
Stage 2: Juvenile: Tegrum at this stage of development are typically placed into a host species. They will initially suppress the host's immune systems but once integrated into the body and "camouflaging" themselves to seem a part of that body will strengthen it ten-fold. A Tegrum host is virtually immune to disease and all but the strongest toxins.
At this stage, the Tegrum cannot control its host, though the two can communicate. By this point, a Tegrum is already considerably intelligent and will sample the host's memories for valuable experience and learn as the host does. As it gains nutrients from the host, it will secrete chemicals to improve the host in addition to improving the immune system. At this stage, the primary secretion slows cellular decay increasing the host's lifespan. The host also becomes more resilient.
Stage 3: Mature: A mature Tegrum is essentially non-removable, except by its own free will. It can temporarily gain control of the host, typically done to share its wealth of knowledge with others, though also because they have learned to control the host's body faster than the host. Fibrous nerve endings have spread throughout the host and a Tegrum controlled host can be dangerous in combat.
By this point, the Tegrum's secretions have made the host heartier and more survivable in combat situations. Combined with the advanced weaponry developed by the unified creatures, the host creature is a strong warrior.
Stage 4: Venerable: By this point, the Tegrum itself has lived no less than 500 years, often in the care of multiple hosts. Afterall, they only slow the growth process of the host, not stop it. A venerable Tegrum is capable of controlling the host indefinitely, though few do. Free will is an important concept in the Tegrum social structure. Though the species has begun to recognize that free will may need to be sacrificed in favor of survival.
Should a Tegrum of this stage deign to enter combat, it is nigh unstoppable. The host body has been hardened and made incredibly resilient. A human host at this stage would be on par with an Astartes.
Acceptable Hosts
Due to the requirements needed, not all species in the galaxy make good hosts for a Tegrum. Their Blanks were the most compatible species from their glaaxy of origin. The most compatible species in this galaxy are Human, Eldar and Tau. Species such as Necron, Dark Eldar and Tyranid are completely incompatible due to their unique physiology or brain structure. Other Tyranid will actively seek to destroy a Tyranid with a Tegrum in it. An Astartes is considered a prime example of a host creature but only a Venerable Tegrum can hope to survive the implantation process, do to the Marine's formidable immune system.
Because the primary means of host interaction is through electrical impulse, mechanical bodies have begun to be produced for Mature Tegrum. These bodies are very sturdy but difficult to maintain. The nutrients normally received from a host must be artificially introduced and most Tegrum would prefer not to have an artificial host.
Another growing trend among the Tegrum is to implant into a non-sentient life form. Donarian Clawed Fiends and Fenrisian Wolves have been captured for this purpose. It has been found that even a Juvenile can temporarily control a host of this nature.
Technology
Tegrum technology is designed to be utilized by their hosts more than themselves. Most of it is therefore designed with humanoids in mind, the artificial host being an obvious exception. Tegrum would prefer to negotiate and achieve their goal of uniting all they encounter. But they have encountered resistance to this before and possess very destructive weaponry. Such weaponry has cowed enemies into submission.
Tegrum Consciousness
The Tegrum are not overtly psychic but are all linked mentally. Those in close proximity can communicate telepathically. Furthermore, if multiple Tegrum are present without hosts, they can fuse into a larger being to better protect themselves. If a large enough group fuses, they will create a monstrous creature, its multiple minds resulting in a kind of madness leading to violent rampages.
This obviously needs some work but I didn't want to wait too long before posting. Aside from their Artificial Hosts and "Monster Blobs" the Tegrum would not be seen on the battlefield. The "Monster Blobs" would appear after a unit was destroyed(with limitations) and likely have Rage. They are carried in a host species that gains benefits. Hosts with Juveniles would likely gain FNP, while those with Matures would have higher toughness and initiative in addition and those with Venerables would have further bonuses to the model's/race's base stats. Obviously, these would make the units cost a little more and there would need to be limitations on whether or not a Venerable could inhabit Elite or only HQ units. I have too many ideas to post since this is just suppossed to be brainstorming and I'm getting tired. Let me know if you want anything clarified.