The species I want to see the most in Mass Effect: Andromeda is ….. the Rachni. I don't care how BioWare does it; retconing their survival, a second Rachni ship sent out during the end of their war with the Council races, etc; but this is one alien that needs to make it into the new game for several important reasons.
1. Because I like them. The Rachni, the Elcor, and the Hanar are the least anthropomorphized out of all the aliens present in the game, and out of the three of them, the Rachni are the only one that hasn't been reduced to a joke or meme status.
An alien society based around a hive mentality (not a hive mind like the Geth), and their interactions and dialogue strongly influenced by color and sound, makes them one of the more 'alien' aliens in the game. They desperately needed more screen time during the trilogy, but they were swept under the rug, it's time for that mistake to be corrected.
2. If you are going to attempt to colonize an entirely new section of space with no pre-established infrastructure, if you are going to be exploring possibly hazardous planets, if you are going to need able bodied soldiers in the event of hostilities then Rachni are the only real choice. They are incredibly adept engineers, they can establish a colony in the span of a few days, birth combat capable young in a matter of hours, and they can survive environments in nothing but their exoskeletons; environments that even the hardy Krogan require sealed suits to operate in.
Trust issues with the space bugs is a moot point when survival of society is concerned, but if one must point fingers at their actions during the Rachni Wars, and the events in the first three games then you also have to consider how much the galaxy was and is willing to trust the Krogan. A species that has caused the same; if not more (there is no mention of the Rachni rendering three garden worlds completely un-inhabitial during their hostilities); damage to the galaxy at large. Out of the two species, the Rachni have admitted to their atrocities, and have made attempts to try and establish inroads with the galaxy by way of providing aid for the war effort. The Krogan have not, nor have they, as a whole, even attempted to acclimate to galactic society; instead they blame all of their woes and hostility on the Genophage, and when the Reapers invaded they would rather hold the galaxy hostage to force a cure out of the allied species than helping to fight the monsters coming to kill everyone.
Lastly, the Rachni's 'vulnerability' of their Queens is a moot point as well since the only species that was able to 'assume direct control' of them was the Leviathans; the resident God-Tier species; even our hero Commander Shepard was unable to resist the Leviathans' mental assault. As far as the Reapers go, the Rachni have proven to be (essentially) immune to indoctrination effects from the Reapers; why else would they have to keep the Queen shackled if they could just break her mind?
3. Because the Rachni are one of the few 'alien' aliens left in the game, and while BioWare will add new aliens for us to encounter in Andromeda, how many of them are going to be aliens vs just another human in a rubber suit? BioWare can tell good stories, but their track record when it comes to adding in new non-human elements is sorely lacking.
I mean what species was added after Mass Effect 1 that had that same level of alien-ess as the Rachni?
The Geth? Expanded on an existing race from the first game, and then 'fixed' to be more like us in the third game anyway. The Collectors? No character there, just mindless meat puppets of the Reapers/Harbinger. The Leviathans? Reapers 'Skin Edition' Etc.