Like many here have echoed, OP is a lost cause. It's been losing money every year and it's on tax payers dollars. They even spent over a million to fix the pipes for the water park and just after it was finished, the park closed down indefinitely. There is no indication when OP will ever reopen again. If it's going to sit bare and unattended, might as well make use of it and have someone occupy it so they will help pay taxes on the land. The condition would be to keep some of the places as it is for attraction such as the cinesphere, etc. And keep a portion of it open for children's water park so that families can enjoy it too. Also conditions such as having them help pay public transit to the location, have parking stacked instead of sprawled, build a classy entertainment area, etc. If there's going to be a casino built anywhere in the GTA, Toronto will probably end up paying for the brunt of the social issues created, so we might as well try to make money from it.
If there's any indication that there's a plan to save OP and make it a great family place, I would be against a casino and entertainment complex there. But nothing has surfaced for many years and it looks like there's no future plans for it either. The government is focusing on revitalizing the east end (west donlands, east bayfront, portlands, regent park, etc).
There's a big difference between Canada's Wonderland and Ontario Place. Wonderland is privately owned. OP is publicly owned. If OP was sold to someone as an amusement park, it would probably be run better.