I've seen it said online by EDI route development employees that 9 out of 10 passengers travelling to the US do so via a UK/European hub. It is this market that they are targeting by launching new US destinations.
LAX have also in the past issued a wish list of destinations they'd like to see served, with Scotland being one of those shortlisted. I think the most likely to operate that is probably Virgin, although they dont seem to be doing overly well at EDI at the moment, so I think expansion might be some way off.
I think the next US flight we will see will be AA rejoining the market, probably in 2 years time when they get the A321XLRs. Possibly Delta to Atlanta.