Unless the Ontario Line is the first stage of what would in essence be a BART system for the GTA, I think it risks becoming another SRT-like orphan technology. For it to be truly successful, Metrolinx needs to commit to building Ontario Line-compatible tracks within existing rail corridors. Without that commitment, all you're doing is building another subway line that's incompatible with the existing subway, and creating a NYC-style IRT/BRT situation.
The corridors that are the prime candidates for this treatment are the Milton, Richmond Hill, and Midtown lines. None of those lines are included in the RER program, and they all would require some flexibility within the corridor in order to shoehorn them in. The advantage of the lighter rolling stock is that it would allow the corridor to go above, below, and around the existing rail lines, fitting in wherever there's space.
If this is the beginning of the GTA's version of BART then I'm sold. If it isn't, then I seriously question the need to introduce a new technology into the mix.