Well like I said before if they're from a different dimension, it'll probably be one that's linked to the normal one not a complete alternate world. Think the Wolf Among Us :P 
And yes Aliens are too big a thing to have this early. I'm thinking full-on AI will be introduced sooner than Aliens would be maybe next thread. 

You could have somebody who was turned into a metal man by the Awakening just without all the fancy electronics. Or you could have some kind of machine possessing metahuman. Maybe a technopath who lost their real body and moved into a machine one. 

Architect, Heya. It's good to see another new face. As of this thread the basic bullet list of important events is:

1. A League member named Furious was kidnapped by Legion in a mission.

2. Another League member named Sixgun infiltrated the Chicago Outfit who are starting to push back at Legion, they actually managed to kill a Sorcerer.

3. Legion managed to capture most of the Outfit boss Marconi's family, but some of their own were captured by the League. 

4. Legion started a prison riot and mass breakthrough in Cook County Jail, where the majority of supercriminals for most of the nation were held. 

5. The League and the government managed to secure the prison and put down most of the escapees, and capture two Legionnaires, but the majority of themaximum security criminals and perhaps a few others escaped and a few League members were killed, the first casualties the organization has experienced. 

6. The League is currently fighting Polaris, a former top-tier hero and friend of Zenith and Olympia turned villain who was released from maximum security by the Legion. 

7. Another League team is striking a Legion owned brothel where the prostitutes are being mentally indoctrinated along with the abducted civilian superhumans. A member called Morningstar found they were holding a ten year old Conduit, a metahuman who can affect the powers of other superhumans. The League is currently trying to extract the Conduit along with the other civilians.