I'll bet you could expect some additional growth via modest up-gauging once the new concourse opens, but that's not going to be huge (50 seaters to 76 seaters at most).
On that last point, how are they building out the gates on the new North concourse? Will they be half height and so only accommodate RJs? Or will they be full height and usable for 737 and up (vertically that is)? I imagine that limited space on the tarmac and a desire to get as many commuter gates as possible in means that there is not enough space between gates to routinely use them for larger jets, at least when adjacent gates are occupied.
I'm pretty sure the design basis is for E175-sized aircraft. They could reconfigure gates in the future to accommodate larger aircraft at slightly fewer gates, I'm sure, but the design is to replace the same number of hardstand locations for the current Gate 35X with jetbridge gates.
They're not half-height, and will have full jetbridge access.
Here's what the planning document says: https://www.mwaa.com/sites/default/file ... 110216.pdf
As noted in Section 2.3.2, the purpose of the New Concourse is to improve the passenger experience by providing contact gates for 14 regional aircraft presently operating from hardstands (i.e., off-gate aircraft parking areas where passengers embark and disembark) and an undersized passenger holdroom. The Embraer 175 (EMB175) was chosen as the design aircraft because it is representative of the aircraft that can be gated at the existing hardstands, is commonly used in the existing fleet, is projected to be utilized in the future fleet mix, and meets all existing DCA operational statutes, regulations and requirements.
The 'operational statutes' I believe is a reference to DCA's slot regime, which specifies a number of slots that may only be used for commuter aircraft - under the FAA's definition, those with 89 seats or less. I also think that AA holds a large share of these slots (as evidenced by their Gate 35X operation). The plan allows them to increase all of those RJs to E175 aircraft once the concourse opens.
In the future, however, MWAA did plan the building to allow for bigger aircraft in the future. From later in the EA:
Consistent with industry practice, the building is being sized to accommodate future flexibility in aircraft fleet mix. Although the current forecast only supports the 14-regional aircraft gates, the airport has identified how the concourse could be used to accommodate a smaller number of slightly larger aircraft