The best option for VIA is to obtain access to the Mont-Royal tunnel. This is obviously what would be best for the HFR and for connection in downtown Montreal.
I am not very optimistic anymore for that to ever happen. I was at first then realized that the infrastructures needed to efficiently mix the REM and VIA trains will unlikely be built at the current time. I don't see how a VIA train can fit within the REM trains when the frequency between the shared portion (between A40 Station and Central Station will be every 2 minutes at rush hour). It is technically doable I am sure, but quite a challenge. Take this challenge in Japan / Germany or China and it would be a no brainer to fix, but here, we seem to be stuck with an old mentality.
An alternative would be to use a new terminal North of the Mont Royal. Perhaps, a the A40 junction station. This will be the new terminus of the Mascouche cummuter line and directly linked with the REM.
This is what I think will happen if ever the HFR gets funding and is underway.
There is already a connection between the Trois-Riviere Sub from the CFQG and EXO connecting to CN (EXO previously known as AMT) :
https://goo.gl/maps/yugzCUm5iYw
I believe this is where the old connecting track was :
https://goo.gl/maps/nMAL8YxHFXT2 It seems to be quite costly to rebuilt, but not impossible.
Ultimately, if the above options do not work. One could see a potential use of the Lucien L'allier station (Old Windsor station) for VIA's HFR trains to Qc-City. Litterally 5 minutes away from Central Station with metro connection and directly downtown...
A short connection would be required if going onward to Ottawa and Toronto.