As a Super Trooper, you belong to a known and reasonably common group of supers, with enough common powers and limitations that many opponents will know what to expect. If in Costume this can be readily identified.
Some aliens have a set of innate powers specified when the race is created. They should make some kind of sense in that a people all displaying these powers and weaknesses could reasonably survive. Be warned that any race a player designs is likely to crop up as villains as well.
Other super troopers are (former) members of powerful organizations with the ability to grant super powers to their members. These powers have a theme appropriate to the organization. For example, Vazhilok Murk Eidolons are super troopers because they share a distinct look and many powers.
City of Heroes Super Troopers include all the villain groups (except conceptually the Outcasts), as well as all Kheldians. An ex-Vazhilok Eidolon can use this limitation. So can an Alien race the player and GM can agree on.