I just recently found a copy of DDOT's 2012 concept plan for the Southern Avenue SE/Winkle Doodle Bridge Project (it was on the PG County Planning website of all places).
This project is to cover the area along Southern Avenue from South Capital Street to Naylor Road, SE. Several designs were considered, all of which included either a 10 foot shared-use sidepath or bicycle lanes, but in some designs it required a combination of the two (bike lanes in some places, sidepath in others). The preferred alternative (PA) includes bicycle lanes for the full length to create a consistent bicycle experience and limit the taking of right-of-way.
There's a linear park between South Capitol and Galveston Street SE (see image below), under which some of the water from Barnaby Run will flow. The rest will flow through a naturalized stream to the east that replaces the trapezoidal concrete channel there now.
The PA also widens the Winkle Doodle Bridge over Barnaby Run to allow for "a generous pedestrian realm that includes space to preserve and celebrate a historic boundary marker stone."
New Winkle Doodle Bridge, Gateway and Linear Park
The intersection of Southern Avenue, Owens Road SE and Bonini Road SE would be replaced by a roundabout as would the intersections of Southern with Chesapeake Avenue and Southern with Mississippi Avenue. Signalization would be removed and bus stops moved.
Mississippi Avenue Roundabout
That all sounds good, but I was disappointed to see no mention of the Suitland Parkway Trail. The SP Trail ends just west of, albeit ~20 feet below, Southern Avenue and connecting it to these bike lanes would create some kind of connection on the east end. In the Google Streetview below, the Suitland Parkway Trail is on the left and Southern Avenue is the bridge.
Unmentioned in the plan is that the south end of Southern Avenue will (might? eventually?) connect to the rehabilitated and expanded Oxon Run Trail and continue south on that to the trail in Oxon Cove Park. From there users can connect to the Wilson Bridge. And Oxon Run Trail will also connect to Southern at 13th Street SE and again at Mississippi Avenue.
According to a recent Ward 8 DDOT Project Update, final design on Southern Avenue starts late next year, with construction in late 2016.