That's the most significant. But there's also the Blue Line extension to Lachine, which like the White Line, was once posted on Metro maps in the trains, with black dots. And don't forget at that time, the east extension of the Blue Line, went to Montreal-Nord rather than Anjou - similar to what was shown on the White Line (though my 40-year old recollections might be failing me).
Yes, the initial plan for the blue line had a 4 stop extension west of Snowdon towards Ville-Saint-Pierre (part of Lachine, it wasn't planned to go all the way to the actual Lachine). The extension east was originally supposed to go northeast to Montréal-Nord, but those plans changed with the idea of building the white line and they imagined a line running along Jean-Talon that would intersect at Pie-IX/Jean-talon. That station was designed to be exactly like Lionel-Groulx, with same platform transfers between the two lines.
Also more recently they were studying both the east and west extensions to the Yellow Line to try and relieve the other two lines - very similar to what's shown on that 1967 figure (north along St. Denis to Sherbrooke, to the west, and then south to Nun's Island) - even with consideration an in-fill station at Bonsecours (which is St. Denis south of St-Antoine). After the downtown idea dropped, they were studying the Longueil extension for a while. Didn't they announce it at least once?
Western extension of the Yellow line was a 3 stop extension from Berri-UQAM to McGill. The goal was to eliminate the transfer at Berri-UQAM and relieve the green line downtown segment. There has never been a serious study on bringing the yellow line further south than that (prior to the REM the AMT was studying a light rail link from Gare Centrale to Brossard).
The Bonsecours station is a myth. It was never planned or considered by anyone. The tunnel there is in one of the steepest slopes in the entire network, and building an in-fill station there would require building an entirely new tunnel alignment.
The eastern extension to Longueuil was announced a few times yes. In the early 2000's they studied the possibility of it running under Chemin de Chambly all the way to St-Hubert train station, and in 2009 the alignment was shifted east along Roland-Therrien blvd. It was always planned as a 5-6 station extension.
And of course there's the two-station extension of the Orange Line to Bois-Franc - which isn't as major. I can't remember if that was shown on the early 1980s Metro Map or not.
It was shown on maps, and it actually would have ran to Gouin boulevard (with stops at Poirier, Bois-Francs, Salaberry and Gouin).
Now that I laid it all out here, it is kinda significant, as all these represent 28 stations (a 38% increase from the current/planned 73 stop count). 12 of those are from the White line plan though, which is being replaced by the BRT (we'll see how good it is soon). I am also ignoring Line 3, which is basically being taken over by the REM).
As for future needs... Honestly if we were to build another east-west line, the best solution would be to build a rapid transit line along the CN right of way north of the city from Pointe-aux-Trembles to Côte-de-Liesse REM station. It would serve the northeastern part of the island and give easy access to downtown through the REM.