Given the practical limits on PHL departures for a number of years it'll either be offering more departure times or a shift of capacity - at least to some degree. If both PHL and CLT can support a particular European flight it could be an increase in frequency to two from one flight per day (or 3 from 2).
The 757 is getting pretty old - it's either get newer used 757-300's or replacements. There's not a good replacement right now unless the A321NEO will gain enough range. Or the 737MAX, which AA has ordered (and if the merger goes through).
The 767 lie flat on a slant seats might work in a 2 x 2 configuration - it just depends on the width of a 2 seat unit. There's also the cost to consider - is it worth it to spend the money on what is really a temporary airframe - since the 757's will need replacing by 2020 or so.
As for post-merger, flights to *A hubs will need to support themselves - no *A feed on the other end. It may be that a combined AA/US would have better feed to CLT/PHL/JFK and the flights could support themselves. Only time and a merger will tell...
Jim