Reading the Paris Metro Handbook (yes, I continue my tran nerdery), I'm becoming increasingly puzzled as to why lines 3bis and 7bis have not been combined, despite this having been planned for a long time.
For the uninitiated, both _bis lines were created when their parent lines were extended in other directions, and for operationa reasons the branches were split off into separate lines. Both are quite short, 3bis just 4 stations (1,3 km) and 7bis just 8 stations (3,1 km), and their eastern terminuses (Porte des Lilas and Pré Saint-Gervais, respectively) are just 700 metres apart.
Well, turns out that in addition to the above facts, rail tunnels connecting the two lines directly already exist, and an unused station (Haxo) exists in them. A shuttle service has even been provided between Porte des Lilas and Pré Saint-Gervais. This would be an exceptionally cheap way to provide additional connections in the city, yet the realisation of it has been postponed indefinitely.
(For more extensive and expensive future plans, I would extend the combined line to Gare du Nord in the west and Gare de Lyon in the south.