Everybody has seen the amazing Suicide Squad trailer by now and everybody has also seen the fantastic Batman V Superman trailer and we see in the Batman V Superman trailer a Robin suit in a glass case which is probably a shrine to Jason Todd that Batman decided to put up in memory of his killed side-kick and we all assume that Jason Todd in this cinematic Universe was killed by the Joker but if we think about it for a second it would make sense that actually Jason Todd is the Joker!
What if Jason Todd was kidnapped by some villain and then brought to a facility where he was tortured and turned crazy and ended up becoming the Joker. This is a very interesting idea because we haven't really seen this in the comics so it would be completely new to us. Also what if the villain who tortured him drew a bunch of tattoo's on him as a process of torture which would explain why he has the tattoo's.
If you look under his eye there is a J which could be a reference to his name Jason Todd? Maybe the reason why he has does replacement teeth is because the villain beat him with a crowbar like the original Joker did in the comics and it knocked all of his teeth out? In the Batman V Superman trailer we can see Bruce Wayne looking at an article which has writing on it which seems very similar to the Joker's writing and it is written
"You let your family die"
Normally the Joker does not know who his family is and Jason Todd knows who is family is and maybe it's not a reference to his parent's murder but Jason Todd's symbolic murder. It seems like Batman has not been Batman for a very long time which might be because he was traumatised that he couldn't save Jason Todd. Also the writing that is featured on the Jason Todd shrine is obviously something the Joker added and it would make sense that after Jason Todd became crazy he left a little message for Batman.