They do. I don't think it's finalized yet, but once it is, it should be published on the website. The last master plan dates back to 2013, so they're due.
They came up with a plan in 2018 (
this one, slide 15/16 have good renderings), but quickly abandoned it once they realized the access roads weren't enough.
They announced a new plan last April (
this one), and the rendering clearly shows an expanded road network leading to the terminal.
No official document of the plan so far, but it should come out soon I guess.
The footprint of YUL (plus the runway layout) is similar in size and shape to that of LHR, and Heathrow handled 79 million passengers and over 450,000 movements last year. So the footprint is not the problem. How they develop(ed) that footprint, is. So far, their plans were garbage. They seem to be on the right track to rectify it. Let's see.
It doesn't help that Air Canada and Bombardier are sitting on prime real estate, in the middle of both runways. So YUL will never be able to handle 80 million passengers. But it doesn't need to either. A terminal building capable of handling 40-50 million passengers will serve Montreal's needs for the foreseeable future.
Runway layout and capacity isn't a problem. Plenty of room there for additional capacity.
You'd need to inject a ton of money at YMX to make it a viable replacement for YUL. Significantly more money than at YUL. It's not going to happen. ADM made their bed at YUL, now they have to sleep in it.