With GO planning increased service Niagara Falls ON once this thing is over.....it would make sense to terminate the Maple Leaf at the Border.
Passengers arriving on the Empire Service would clear Canadian CBSA in a joint facility in the new Amtrak Station on the US side then board a GO Train for Toronto. In the opposite direction....a couple of GO Trains a day could be extended to NF NY where passengers clear US CBP then continue on the Empire Service.
And a joint pre-clearance facility seems to have the backing of Sen Schumer (article from last year):
Schumer to Canada: Implement pre-clearance for Amtrak trains
With a little schedule coordination there could be a couple of services a day each way between Toronto and New York.