Fans have a lot of reasons to be excited for Mass Effect 4. While the Mass Effect series is not above criticism, it has one of the most fleshed-out settings in science-fiction video games. A big part of that is the comprehensive lore behind each of the series' alien species, which covers thousands of years of galactic history. Though Mass Effect's worldbuilding isn't perfect, races like the Turians, Asari, Salarians, and more feel fleshed out and very believable.
However, the Milky Way is home to multiple sentient species that have never played a significant role in the story. Some have bit parts in the Mass Effect games, while others are only mentioned by outside sources, and these unseen alien races could appear in the upcoming Mass Effect 4.
Mass Effect 3’s Raloi Species
The Raloi are a feathered, bird-like species native to the planet Turvess. Mass Effect's hulking Elcor were aware of the Raloi's existence for some time, but never shared that information with the rest of the galaxy Possessing a mid-to-late 20th-century level of technology, the Raloi launched their first space telescope in 2184; two years before the events of Mass Effect 3. The telescope detected an Asari cruiser passing through the Raloi's home system, leading to a quick contact with the Raloi eventually sending an official delegation to the Citadel.
However, the Raloi could not have picked a worse time to reveal themselves to the rest of the galaxy given Mass Effect 3’s Reaper invasion begins only a few years later. Realizing they had no means to resist the technologically advanced invaders, the Raloi withdrew from the Citadel and destroyed their own satellites in the hopes that the Reapers would ignore them. Mass Effect 3 doesn't reveal what happened to the Raloi, but it's conceivable that they managed to survive, and could become an ideal candidate to appear in Mass Effect 4 as a result.
Mass Effect 2’s Virtual Aliens
Mass Effect fans learned of the Virtual Aliens through the Cerberus Daily News updates on Mass Effect 2's Cerberus Network DLC. Roughly 8,000 years before Mass Effect takes place, the Virtual Aliens discovered that their home star was about to go supernova. As they didn't possess the technology for faster-than-light travel, one billion individuals chose to upload their minds into a virtual reality onboard an AI-piloted starship. The ship arrived in Salarian space in 2185, seeking help for its failing power source. After a tense back-and-forth where the Council's fear of AI nearly resulted in the Virtual Aliens' destruction, they established peaceful contact with the rest of the galaxy.
Since they only exist as data, only the Virtual Aliens know what their physical forms looked like. However, one of their more interesting features is the ability to swap bodies with individuals of seemingly any other sentient species. In fact, about 400 people from Mass Effects' various races volunteered to upload their minds into the Virtual Aliens' simulated world, with an equal number of Virtual Aliens inhabiting their physical bodies. It's unclear what became of the Virtual Aliens after parting ways with Citadel Space, but they are presumably still out there in the galaxy somewhere. Considering Mass Effect 4 may focus on the artificially intelligent Geth, this might offer an excellent opportunity to check up on the Virtual Aliens.
Mass Effect 2’s Kirik
The Kirik is another race from in Mass Effect 2's Cerberus Daily News. The beetle-like Kirik may not be fully sentient, but they are more than just animals, possessing a surprising degree of intelligence and natural Biotic powers. Both are noteworthy since scientists in the Mass Effects universe previously believed arthropods lack the necessary neural pathways for either. The Kirik's unusual abilities made them a prime target for uplifting into a sentient species, so Mass Effect’s Citadel Council dispatched a team of experts to judge the Kirik's intelligence. The results of their evaluation are unknown.
It's possible that one of the more advanced races chose to uplift the Kirik between the events of Mass Effect 3 and Mass Effect 4. Alternatively, perhaps the Kirik will have more surprises for the rest of the galaxy if they return in the upcoming title.
Mass Effect 2 and 3’s Yahg
The Yahg actually do make an in-person appearance in the Mass Effect trilogy. Players encounter one Yahg as the titular Shadow Broker for Mass Effect 2's Lair of the Shadow Broker DLC, and there’s a nameless Yahg in Mass Effect 3. However, the species has never been properly explored in the Mass Effect games.
A species of towering humanoid apex predators, the Yahg possessed a 20th-century level of technology when the Citadel races discovered them over a quarter-century before humanity's First Contact War with the Turians. The Citadel attempted to contact the Yahg, but the latter were insulted when the representatives greeted them as equals instead of honoring them as superior. In retribution for this perceived slight, the Yahg massacred the entire delegation and the Citadel Council subsequently cut off all contact, banning travel to the Yahg homeworld of Parnack.
However, that did not stop the Salarians from attending to secretly uplift the Yahg before their efforts were interrupted by the Reaper invasion. Notably, the Reapers ignored Parnack, as the Yahg hadn't yet reached the necessary level of technological development. Given this and the Yahg's unusually high level of intelligence, the aggressive species might have developed spaceflight by Mass Effect 4. The introduction of a power-hungry species with the strength of the Krogan and the intelligence of the Salarians would certainly be one way to shake things up.
Mass Effect 4 is in development.