Let me start by covering a few things raised by other people. Firstly, the show has not ignored a plot hole about the daggers killing the vampire who uses them. It kills normal vampires but it does not kill Klaus, it would be a bit silly if the dagger being plunged into his heart could not kill/incapacitate him but him using it on someone else could..., now i know in a previous episode Rebecca stabbed klaus with a dagger and she didn't die, two possible explanations, firstly, it didn't affect her because it doesn't work on klaus and therefore the other part of the dagger doesn't work either or the dagger does kill the vampire who uses it but that rule not only doesn't apply to klaus but does not apply to all the originals.
now, Jeremy leaving? is it forever or just a while, it all depends on the reasons behind it, having not read the books i don't know but there are 3 options, he is gone for good because the character wasn't adding enough to the show, he is gone for a while and will be brought back as part of a plot twist, or he was written out permanently or temporarily at the actor's request for some reason. I hope he comes back.
Now, to my main point, the coffin which will not open. The idea seems to be that whatever is inside it is the key to defeating Klaus, or that seems to be the inference. Now, taking a few things into account, the fact that Klaus,presumably, has had the coffin sealed with magic, secondly, the dreams bonnie had where she sees "klaus" in a coffin with a necklace similiar to elena's, i am not 100% sure it is a different one but i think it is. Here is my wild prediction.
Klaus is a twin and that his brother is in that coffin, i know it is a crazy idea but it just popped into my head,it would explain how the contents of the coffin could be used against klaus because his twin would also be a hybrid and therefore would be as strong as him.
Secondly, it would raise a few interesting questions, for example who killed their mother? klaus or the other one. also, we know that klaus hasn't always been a bad guy, in the flashbacks to when they were human he seemed almost normal, so the questions would be which "klaus", obviously they are not both called klaus, but which one did various things throughout history and which did they swap places at any time.
Now, the most likely reality is that next week they will open the coffin and I will be proved wrong and all this typing will have been for nothing.
one other theory that popped into my head is that the coffin contains a witch, ayana perhaps, but that begs the questions why would klaus keep it.