Hulk was a MCU movie with an A star cast and it bombed. DCEU will be 5/5 for 100 plus opening weekend movies (probably 6/6 after Aquaman) so I wouldn't put it past a Deathstroke movie making decent profit, especually if the movie has a lower budget. Cant say these things until the marketing starts.
Not really a good comparison, seeing as how Hulk came out right at the start, and Ant-Man came out in a post Avengers world where you could literally say to someone "it's connected to Avengers", and they would go and see it when they wouldn't have otherwise.
It might make a profit, but that's not the point. He's not going to get the buzz he could get until WB properly establish the universe.
You're talking about establishing a shared universe, right? Deathstroke doesn't necessarily need to be apart of that shared universe. It can be a movie all on its own. And besides, it won't be coming out in the near future. Who says the universe won't be established by then?