I really hope and think that:
For the first third or so of the film, The Joker will be a member of the Suicide Squad. He will have been captured prior to the film's opening by the Batman and we'll see and hear all about his fascination with him.
He'll also love the fact that his head can blow at any moment and will test Waller near her breaking point to push and detonate the implant in his neck, blowing off his head. Her reluctance and the outlandishness of the idea will make him doubt that the implants even work.
While working for Waller as a member of the Squad, he will secretly be devising his own plan (with Harley's help) and when the moment is right, he'll flip the script on Waller.
The scene I have in mind to show how crazy he is?
Right as he overthrows Waller -- and before her team of guards storm in on the Squad to apprehend The Joker and put him down -- he gets ahold of the detonators for the whole Squad. He'll use them to keep Lawton and the rest at bay. Doubting the implants are actually explosives, and not caring if they actually are, he'll volunteer everyone for a game of Russian roulette, joking that KGBeast should have the advantage. He'll shuffle through all the detonators (including his and Harley's) before throwing away each one to the floor until he is left with only one in his hand. Harley will have second thoughts, beginning to plead with him not to do it, and he'll explain to her why it's funny before pressing the detonator, killing a member of the Squad with a blast before escaping -- giving the Squad a vendetta against him and Waller a target to send them after.
-R