Mass Effect: Andromeda has a notable lack of new alien species, despite taking place in a totally new galaxy. All in all there was only one companion from an alien species not introduced in the original trilogy, Jaal the Angara. The fact that players knew so little about Jaal and his species helped make him one of Andromeda's most popular squadmates, encouraging players to get to know him as a window into the series' brand new setting. Mass Effect 4 can learn from Jaal's success.
Although in the end Andromeda introduced just two new organic alien species - at least, two that actually made an appearance in the game in-person - there were originally plans for far more. In fact, Andromeda could have had up to 10 new alien races, but many were cut for budgetary and cosplay reasons before the game's release. Mass Effect 4 should bring some of those cut concepts back.
Mass Effect's Aliens
Mass Effect 1 was able to take advantage of its setting's species in a way none of the games have been able to replicate since. In the first game, each alien companion was a window into their own respective species, and by learning about their species the player also ended up learning far more about that character as an individual at the same time. Even the player's human companions give them a window into the history of humanity in the Mass Effect timeline, making it worthwhile to get to know them to learn more about the galaxy.
Mass Effect 4's trailer implies that Andromeda and the Milky Way will be connected in some way, with the next game taking on species and plotlines from both Mass Effect: Andromeda and the original trilogy. In the background of the trailer's final shot, the Mudskipper ship previously seen in an image released by BioWare can be seen in the background, behind Liara. In the original image the Muskipper appears to have an Angara crewmember, implying that the Andromeda Initiative somehow made it home. This is also implied by the inclusion of a shot of both the Milky Way and Andromeda at the start of the trailer.
Mass Effect 4's Opportunity
If the Andromeda Initiative somehow managed to set up a link between the Heleus Cluster and the Milky Way - perhaps by building a Mass Relay - BioWare has an opportunity to introduce more alien species from Andromeda which were not featured in the last Mass Effect. Mass Effect: Andromeda only takes place in a small part of the galaxy unlike the original trilogy's relationship with the Milky Way. Andromeda sticks almost exclusively to the Heleus Cluster, but creating a link between the two galaxies the next game could see inhabitants from all across Andromeda suddenly start appearing in the Milky Way.
In many ways, it seems like Mass Effect 4 will be turning back to the series' past, bringing back Liara and possibly Shepard. However, it can't just rely on nostalgia. Mass Effect 4 needs to inspire the same sense of wonder, creating a universe full of unknown species who feel worth meeting just like the aliens in the first game. If nothing else, the next game could start exploring some of the Milky Way species that never really got their time in the spotlight or as companions during the first three games, like the Batarians, Vorcha, Elchor, and Volus.
Mass Effect 4 may be bringing together elements from both the original trilogy and Andromeda, but it will need to be more than the sum of its parts if it's going to get the series back on top. New alien species could be a huge part of that, and hopefully some of the aliens cut from Mass Effect: Andreomda will finally get to make their debuts in the next game.
Mass Effect 4 is in development.