Well Toronto already has something similar to that.
http://en.wikipedia.org/wiki/Toronto_Islands
If this Humber Island can help keep Sunnyside Beach clean and allow people to swim in it again then it would be a huge plus. Even if the island is kept as nature only. Hopefully a bridge can connect it for pedestrians and bicycles.
As for an Ile Ste-Helene type of development, there's always Ontario Place for that as well as the Toronto Islands which are barely developed to begin with.