Amtrak is already planning to add a second daily round trip from Montréal to New York once the customs preclearance facility opens in Montréal. My suggestion is that one of the two trains should run overnight, so that it can capture demand currently dissuaded by the long travel times.
The Montrealer operated as a sleeper train up until it was cancelled in 1995 (my wife and I were booked on it for our honeymoon and had to pivot when it was cancelled a few months before our wedding). Its replacement (the Vermonter) runs as a daytime train, so it could be a challenge to have it revert back to running overnight, as that wouldn't serve Vermont very well (who pays to subsidise the train).
As I mentioned in my previous posts, I'm assuming that pre-clearance facilities are built in Montréal and Toronto before the corresponding overnight services would be introduced. Stopping at the border to clear customs late at night or very early in the morning would probably make the services annoying enough to not be viable.
The Toronto services would therefore be quite far in the future, with Detroit Central station and Niagara Falls Ontario station being higher on the list for pre-clearance facilities.
I just don't see preclearance ever happening in Toronto. It is just too far from the boarder for trains to operate without any other stops in Canada to allow domestic travel. Having said that, I think changing the Maple Leaf to a sleeper train makes a lot of sense, even if they have to stop to clear customs in Niagara Falls. A departure of around 20:00 would allow customs to be cleared by 22:00, still allowing a somewhat reasonable departure from Buffalo. Northbound, arriving at the boarder around 7:30 would still likely allow the train to arrive in Toronto by 9:30.
This schedule would allow the train to be an early morning train to Toronto and NYC and an evening train out of both Toronto and NYC. Having an X configuration with trains to/from Montreal and Boston would further enhance its flexibility.