Will initially show 50 states, probably colored by which party controls the most seats in them (or split in some way to represent the percentage of seats for each party)
Hovering over a state in this initial view will show how many seats each party controls (and possibly the popular vote totals for that state?)
Clicking on a state will zoom to reveal the individual congressional districts in the state, similar to the NYT presidential map's method of showing counties
Hovering over a district will show margin and voteshare like every other map source
A house of representatives map: