Last year I asked for a BW Parkway trail. That's extremely unlikely. I also asked that the WB&A be connected to the Anacostia Trail system, which isn't bad, but what people really want is an easy way to bike to Baltimore. That can be done, but it takes a somewhat experienced cyclist. Expanding the WB&A (and building the still missing bridge) is the best option. The trail has been expanded in the last year - in both cases toward the Patuxent River - but crossing the river is proving difficult due to one particularly cantankerous landowner. Apparently the owner even put up a sign saying that the trail will never cross the Patuxent, when the PG side first opened. Let's assume that, as promised, the river is crossed. And that then the South Shore Trail is built. You'll then be able to bike from Glen Dale to Glen Burnie on one basically contiguous trail, a great boon to all those Glen Dale to Glen Burnie commuters. [Of course, you'll have to go to Odenton, and then to Annapolis to get there].
The WB&A is named for the WB&A Railroad. That railroad was actually three railroads; one from Baltimore to Annapolis (part of which is now the B&A trail), one from a point on the B&O railroad to Annapolis (part of which will be the future South Shore Trail) and one from downtown DC to Baltimore (part of which is now the WB&A). South of Glen Dale the MLK Jr./George Palmer Highway (MD-704) was built on the WB&A right of way. North of Odenton WB&A Road is on the right of way all the way to BWI airport.
The WB&A should be expanded in both directions.
North along WB&A road is relatively easy. From Odenton Road, where it now ends, to the southern end of WB&A road is a mile and half, with one difficult crossing of the Patuxent freeway, but from there to BWI, and the BWI trail, WB&A Road is more than suitable for cycling for it's entire 4.5 mile length. A few signs, perhaps some paint and you're good to go. This would create a loop between BWI, Annapolis and Odenton.
South is a little more difficult. The MLK highway is not a good cycling road. So something more like the Custis Trail will be needed. But the payoff would be huge. There is no really good way to get through that area by bicycle (here are two routes suggested by Mike Plakus - longer and shorter). The 6.2 mile extension would connect the WB&A with the Marvin Gaye Park (nee Watt's Branch) Trail and thus with the District. It would serve many ignored areas like Fairmont Heights, White House Heights and Glenarden. It would pass within a mile of both Redskins Field and the New Carrolton Metro.
And of course, when that's all done (creating a backwards four with points at Annapolis, DC and BWI) it won't be too hard to extend the trail to Baltimore. The light rail runs along one railroad right-of-way, but a second sits right next to it. It's only seven miles to Camden Yards.