Article I Section 8 grants Congress the power to establish a federal district but does not require it. Therefore, Congress could presumably pass legislation to eliminate the current federal district. The 23rd Amendment says that the district constituting the seat of government shall be granted electoral college votes. This would presumably be rendered moot if the federal district were to be abolished and its territory returned to Maryland. There is no reason that a federal district is even necessary. The reason was originally that it was feared that a weaker federal government might be intimidated or influenced by a stronger state government in which it was located. This is no longer a realistic fear so eliminating the district would be reasonable. There already are plenty of federal buildings in Maryland and Virginia (most prominently the Pentagon housing the Department of Defense). Why not just eliminate the district altogether?
The alternative would be to enlarge the district by adding to it the suburban areas in Maryland and Virginia. As a state it would be reliably Democrat but Virginia and Maryland might be competitive in national elections. Of course the 23rd amendment would have to be repealed.