Alrighty...
The colosseum could be merged with the convention centre, converted into a condo with an enclosed atrium, rental apartments, homeless shelter. Converted to a warehouse, or a greenhouse like the Muttart Conservatory or agricultural featuring locally grown exotic food. Indoor playground, religious building, rec centre with an indoor velodrome. Disco / rec room, casino. Office building, hotel, garrison / command / training centre, paintball arena, museum of industry featuring heavy equipment in all shapes and sizes. Walmart, Home Depot, Cabelas, Canadian Tire, Save on Foods, shopping centre. Anything related to education. Skyscraper (of course a skyscraper suggestion was going to be added, it would be sacrilege if I didn't!), Cirque du Soleil venue, Amazon headquarters, Skateboard park, go karts, baseball batting cage & golf driving range. Virtual reality simulator. Research lab, fire/police station, jail , hospital, bank, vault, morgue. Factory, power generating station, transit garage, park & ride parkade. Wildlife sanctuary, aquarium.
That building could be anything above, and more; yet our mayor says it should be demolished.
Now the 'experts' will chime in and say things like: "it costs too much, there is mold, asbestos, bird droppings, building codes don't allow, it's not viable, lawyers won't allow, too much red tape, it is ugly, it's awkward, it's risky" etc.