I'd extend it to Gravesend, taking over the line from Abbey Wood to Slade Green (calls at Belvedere, Erith and Slade Green), then entering a tunnel, calling at Dartford beneath the current station, then branching away from the current line, calling at Stone (close Stone Crossing), Bluewater, then rise to the surface and call at Ebbsfleet Valley (Swanscombe would close, this station serves the other side of Swanscombe) and Ebbsfleet International (platforms perpendicular to the main platforms, they would go through car parks A and B, Northfleet station would close, a walkway from the station to Disneyland (when it opens) will be provided), then it would run to Gravesend, where the station buildings on the south side of the station will be demolished for an extra terminating platform so there are 2 terminating platforms. All services would run all the way through to Gravesend.
Services on the North Kent Line would all terminate at Abbey Wood with the Thameslink service scrapped and changed to Luton to Sevenoaks (bin off Welwyn peak services). Additionally all services on the North Kent Line would stop everywhere now.
Services on the Bexleyheath and Sidcup lines which used to be rounders can terminate at Dartford with the extra capacity. I would propose 2 platforms at Dartford being for terminating services and 2 for through trains.