Line 5 will be equipped with ATC from day one, with the trains being fully controlled remotely between Mount Dennis and Laird. The drivers will simply be watching for track obstructions and opening and closing the doors. Presumably this could be easily upgraded to allow for fully remote trains with the line being split at Laird or Science station if Leslie was grade separated. Might require PSD. Operating automated would certainly be cheaper operationally, especially as the line is extended to Renfrew/Pearson/further.
As for capacity, the line as it currently stands will likely have sufficient capacity for many years to come, with trains short-turning at Laird/Science Station/Sloan to allow for higher TPH in the higher demand sections of the line. Also platform extensions are plausible as alluded to above. New trains must be low-floor but they could be better designed to use their footprint for more capacity (ex longitudinal seating).