The first fan theory about Jared Leto's Joker was crazy, explaining how he might not even be Joker after all! How he was really a battered, bruised Jason Todd (Robin/The Red Hood) taking extreme measures to get revenge on Batman. This concept of a sudden escalation from normality to insanity reminded me of the events of 'Batman Beyond: Return Of The Joker'. Towards the end of this (incoming spoiler) we see Tim Drake tortured into becoming Joker Jr after being captured by Mr J himself. Although after persuasion from Batman, Joker Jr shoots The Joker, realises he killed 'his master', Tim regains control and comes back to Batman.
This made me think, what if Tim Drake hadn't regained control and The Joker was fully successful in his creation of Joker Junior. What if after following Batman's orders to kill The Joker and realising he had just killed his maker, he hated Batman as a result? Fleeing to cry and contemplate what he'd just done, away from Mr Wayne. Growing up with all these dark thoughts in his head and getting tattoos, showing the extent of his broken mind.
This would also follow a similar theme of 'One Bad Day' as seen in The Killing Joke, where The Joker tries to prove the point of that 'all it takes is one bad day' for a person to go mad.
Overall, I think Jared Leto's Joker will be interesting to see in Suicide Squad, whether he is really who he says he is, or not.