I want to start by saying that Joker is an amazing character and is one of my favourites. Secondly, Jared Leto is great and I think he's perfect for the role. Thirdly, I think the Suicide Squad movie will be a fantastic movie. Finally, and most importantly, THIS IS MY OPINION. Don't go ape shit if you disagree with me.
It's too soon. The Dark Knight came out in 2008 which is only seven years ago. Heath Ledger was the best Joker and I think he always will be. I think Leto has a chance of being as great but it's too soon since The Dark Knight for another Joker. I think that Leto won't be as liked because of the recent memory of Ledger and his death.
Joker is too badass. Joker will not be part of the Squad but will just be part of the movie. If you're going to make a movie about villains and it's going to have Joker in it .... MAKE IT A MOVIE ABOUT THE GOD DAMNED JOKER. I know the animated movie worked with him in it but this is live-action and might not be as successful.