Extending the green line to BWI would interesting as you could link Baltimore and Washinton with Subway/Light Rail.
I agree with the posts that it does not make sense to do this. I measured the distance from the Greenbelt Metro station to the BWI terminals as around 20 miles, figuring the route would follow the CSX ROW and/or Rt. 1 for most of the distance then cut over to BWI, approaching it from the east or NE to get around the runways to the main terminal. That would be a $5 or $6 billion extension at least, probably considerably more with substantial ROW acquisition costs. The Silver Line is a 23 mile extension with a $5.8 billion price tag, but it follows a route passing through multiple major job centers and residential areas to Dulles (and beyond) which do not have a rail access option. A Green Line extension to BWI would be a long slow expensive way to the airport and to connect to the Baltimore light rail line.
If one looks at the map on the cover page of the 2002 Baltimore Region Rail System report available on this
webpage, it shows the MARC Penn Line/NEC as the main higher speed spine the transit lines connect to. The Baltimore Red Line, while changed from the 2002 plan, still connects to the Penn Line at West Baltimore and a new station at Bayview. The Green Line subway would connect to the Penn Line at a new station at Madison Sq and if it is ever extended that far, at Martin State Airport. The plan calls for a Yellow Line extension west from BWI to the BWI MARC/Amtrak station to Columbia MD. Which would make the BWI MARC/Amtrak stop the connection for light rail trips to Columbia, Merriweather Post Pavilion or other stops south of Baltimore. Or a short light rail hop to the airport terminal.
On the DC end, the Purple Line will connect at New Carrolton. Maryland has long planned to make the MARC Penn Line into a regional system with frequent service. There should be state funding coming for incremental NEC upgrade projects. The depressing aspect of the 2002 plan is that the 11 years since then have been spent inching the Red Line project forward which is only now in the later engineering design stages. Progress has been slowed by the lack of adequate state transportation funding, but with the increased tax revenue, MD should be able to consider follow-on transit projects after the Red and Purple Lines are built - if the state leadership is there.
Besides extending or new Baltimore transit lines, those projects could also include extending Metro lines a couple of stops or a few miles: Orange to Bowie, Green Line to Bowie or Laurel, a Green Line branch to the National Harbor, Purple Line LRT to National Harbor, etc. With National Harbor getting a casino, there will be interest in someday extending a transit line of some sort to it. But 10 or 20 mile extensions of the Metro lines on the MD side are not good investments compared to the NEC and connecting light rail options. On the Virginia side, on the other hand, significant Metro line extensions may make sense.