Alright, given that it seems like they are close to being gate constrained in March/April, I will assume not too many more adds in the coming month or waiting until start of May for new routes out of BOS. Seems like the largest wave of adds will come toward the end of 2020 and beginning 2021, coinciding with the European entrance and arrival of A220 and the expansion of terminal E.
Seems like A220 replacing E90 will represent a large increase in passengers. Will Terminal C be expanded to handle all that traffic?
Just taking a look at markets out of BOS and where they could still add to complete their build out. Let's assume we are currently at close to 180 flights by end of October.
NYC - currently 7x EWR, 6x JFK, 6xLGA - I think the growth here will be upgauging JFK. Those will be among earlier routes to get A220s, possibly as an additional rotation after transcon R/T. EWR could add more flights to have better schedule vs UA and especially differentiate from DL (which is at 4x daily). If they can get more LGA slots, they really need more in order to have a better schedule. It's severely lacking.
DC Area - currently at 15x DCA, 5 to 6x BWI. They are at same frequency as AA or maybe even better on some months, so no further increase needed. Even 14 is plenty, differentiating itself from DL at 6x. BWI will need to be upgauged to A220 to compete all cost vs WN. Right now, they get same yield as WN, but E90s costs are dramatically higher than 737. I don't think more flights will happen.
Chicago - Currently at 6x on peak days, AA/UA are at 8 to 10 depending on the month and DL at 5x. If this latest increase is good, they should raise it up to 8x for peak season. Again, another route that needs to be upgauged to A220 sooner than later to compete in economics against UA/AA who are both running mainline here.
Philadelphia - Currently at 8x on weekdays, AA at 12x and DL at 7x. Their schedule here is fine. Based on the LF I'm seeing in Q1, I don't think this route can support more flights. The only way I see them increase here is if DL drops out.
PIT - 6x on weekdays for most of the year, DL at 5x. Again, another route that has too many flights at the moment. I don't see them increase here even if DL drops out.
North Cali - 5x daily, UA at 6 to 7x daily, AS at 2-3x, DL at 1-2x to SFO. Based on recent data, the market has now absorbed the mint capacity as UA retreated to all 757s and DL dropped D1 service. I think it's good time to go up to 6x daily and match UA in schedule on most days. If that's successful, they can even go to 7x daily. It's important they do well in markets like this, especially vs DL in order to win more of the corporate dollar. Their service level to SJC/SMF can remain as is.
South Cali - 4x to LAX, 3x to SAN, 1x LGB, subdaily to BUR. They are doing well enough on LAX, that they can go to 5x. At some point, AA will retreat here due to their losses and B6 will be the clear market leader to LAX. Their schedule to SAN is fine. I don't think it needs to be changed. For other La basin market, I could see BUR switched to A220 if they want to keep it around. Or better yet, move that flight to SNA once A220 is available if BUR continues to struggle. PSP can be dropped.
PNW - 2x SEA, seasonal PDX. I don't think anything needs to be changed to SEA. The market is still absorbing mint. It might take another year or more for them to win over the top corporate dollar here from AS. They need to be able to offer year round service to PDX. I see this as an early market for A220. They can add a second A320 during peak season, but the current schedule is just noncompetitive vs AS.
Rest of West - 1x PHX, 4x LAS, 1x SLC - I think they are maxed out to LAS with 2x mint and 2x A320. The market can't handle more capacity. I would like to see PHX be converted to mint. That will give them a boost and also differentiate themselves and prevent DL entrance. SLC will continue to be noncompetitive. They probably need to convert that to A220 down the road.
Texas - 2x AUS, 2x DFW, 1x IAH - Of all the places that need A220, this has to be among top of the list. DL was smart to unleash A220 on DFW/IAH first. With B6's zero presence in Texas, their way to break out is to use A220's product and economics. BOS-AUS will be among the first to go to A220, since this is the longest E90 route that they operate. They need to probably go up to 4x DFW and 3x IAH down the road to be competitive in these markets. If they are trying to have domestic network that's complete, then the current token flights are going to cut it. I would also add SAT, since it's one of the largest unserved airport from BOS and can be served from JFK. Since they have given up on AUS focus city, they should no longer ignore SAT.
DEN - 2x, UA at 5x and WN at 2-3x - A market that they are doing reasonably well in given their lack of presence there. They are moving to upgauge here to A321 to be more competitive. If that's the case, I don't think any change is needed. for better schedule, they could eventually move to a mixture of A321 and A220 and go up to 3x.
ATL/MSP/DTW - 5x ATL, 3x MSP, 4x DTW - ATL/MSP will also be among the first markets getting A220 imo. It's hard for them to make A320 work and E90 costs are too high. DTW can be upgauged to A220 a little later. I don't think they will add more flights there.
Upstate NY - 4 to 5x BUF, 1x ROC, 1x SYR - the frequencies are fine as they are. I think they will be among the last to still be on E90 since there is already too much capacity there. I could see them adding a second ROC when AA drops out. 1 E90 to SYR is already too much capacity. Even if DL drops out of BUF, I don't think they can add more capacity there.
CLE - 4x peak days, DL at 3x - I think the frequencies are fine. Not really in rush for A220 upgauging.
RIC - 4x peak days, DL at 3x - Same as CLE
RDU - 5 to 7x daily, DL at 6x - I think they can go to 7x for longer period of the year. Other than that, this is already a lot of flight.
BNA - 2 to 3x daily, DL at 4x, WN at 4x - their performance here is fine here on A320. I do think they need to go to 3x daily for most of the year. This another A220 candidate. Not in a rush though.
Florida/MSY - I think they are more or less at right capacity in these markets.
New Markets they should add:
IND - 2x daily with E90s/A220
CMH - 2x daily with E90s/A220
MEM - 1x daily with E90/A220
SDF - 1x daily with E90/A220
MKE - 1x daily with A220
So I think there is quite a lot flights they can still add domestically in addition to the international markets.