This expansion could be good if they put some time and effort into joining with the Fallout Universe in some way. The weight of the social conflict on earth building up to annihilation - perhaps the aliens could lend insight or information that fills in even more of the backstory to pre-war Fallout.
Oh but wait - the main game trivilized nukes, made them common place and ridiculous. It also made bland quests and didn't have the dark humor of the social impact of annihilation.
I guess the expansion will be like you guys say - "Shoot em' up Shoot em' up POW POW!!!"