I’m personally still a strong believer that the VIA Concourse couldn’t be used for ALTO service no matter where the platforms are, even with a modern boarding process there’s gonna be too many passengers who need to pass through the space.
Factoring out lineups, the concourse is almost at full capacity when more than 1 VIA train departure is coming up.
The platform reconfiguration, increase in regional rail service, UP Express terminal relocation, and close tie-in to the future Bremner Concourse will bring a lot more non VIA/ALTO passengers through the concourse too.
The only layout modification that could happen is knocking down some walls around the remaining alcoves (some are to be taken up by new platform entrances) to open up space. I guess you could also remove the washrooms and Tim Hortons, alternative options are close.