If the City does opt to use tolls, and chooses plate recognition technology, I would like to see it somehow linked to the Presto ePurse. What I mean by this is you would have two different "fare" categories: License Plate linked to Presto, and License Plate not linked to Presto. The Presto fare would be equal to the TTC Presto fare, while the non-linked fare would be subject to some sort of plate recognition surcharge (like the 407).
This would have the added advantage of people who normally wouldn't buy a Presto card to get one. That may get them to occasionally try taking transit, since they already have one in their wallet. This would also have the advantage of making it easier for GO to eventually charge for parking at GO stations using the same technology. As a side note, I'd also like to see a Presto option at Green P's.