I like aliens, even the evil ones, I really do.
But in games they just turn out to be humans in a different body with mostly very human motivations.
What is the use of adding them? It will be just a gimmick.
Thargoids are already in. They belong to the Elite universe and I am fine with that. They represent the classic alien danger.
What I would like though is adding an intelligent alien species that is unapproachable and truly alien. I have mentioned it before elsewhere on this forum.
I would like the rare possibility of encountering their impressive far advanced alien ships which completely ignore humans and do stuff we can not understand and can only speculate about. We might see them entering stars for example.
We might discover a huge alien dyson sphere and observe alien ships entering it and leaving it, but we are unable to follow them inside. We might even be allowed to land on the huge sphere, but we can not even scratch it. Or we might discover the systems these aliens live in. We can see the signs of super advanced habitation on the planets, but are held back by invisible force fields from entering the atmosphere of their planets. Sometimes a ship might approach us and then leave again without even trying to communicate. etc. etc.
I want there to be no explanation in the game ever. I do not want us to be able to contact these aliens ever. They should be a total mystery.
We might monetize on them with tourism. We can visit their systems with our passenger ships. But there will be no trade, no exchange of technology or information, no communication at all. We should not even know what they look like.
(I was assuming you were talking about intelligent alien races. It is already confirmed there will be alien flora and fauna, or whatever it is classified as, in the game.)