I think people forget that US is short on mainline a/c since they sold 10 E190's to Republic. That's part of the reason we saw the reduction of T/A destinations this summer. US took those freed up a/c and pushed them down the system to back fill the holes. The only airplanes US has any room to play with are "Express" planes. You won't be seeing more mainline flying out of LGA until we get more mainline a/c and if we do it would most likely be E190 flying. LGA already has most of it's top O&D destinations well served by other airlines.
US also suffers from having the lack of the appropriate sized "Express" a/c for the type of operation they want to run out of LGA. We have CRJ's, E145's, E170/175's flying to destinations that would be better served with more efficient a/c such as the Q400 which either burn less fuel or have more seats to spread the cost. Unfortunately US is hamstrung by the fact of the pilot scope clause which allows 96 86 seat a/c but doesn't allow 76 seat turboprops which could be flown by our subsidiary Piedmont keeping the revenue all for US and allowing scheduling flexibility as needed. (I'm not saying they should give up more but what, in my opinion, would have been better in the first place.)