In Batman Inc. #5 when it shows future damian as Batman and all those crazies running around doesn't Barbara say something like "The Joker's final joke." so if its in continuity doesn't that mean the joker can't die here because he still has to do that later?
If anyone kills the Joker it should be Batman. It should be something like where he doesn't really have a choice. He kills joker or joker kills everyone else type of thing. Its always been the two of them so I think it'd only be fitting if it ended with just the two of them. This way Barbara and Dick could have the after shock wondering if they could've done it or if they should've done it to spare batman. Jason can have closure seeing Batman finally kill Joker. And this could actually be what starts Damian down his Batman path. Using the thought that its okay to break 'the rule' every once in awhile. While all this is going on Batman can be dealing with the fact that he killed the joker. And, of course, the joker could die feeling the ultimate betrayal from batman, knowing that he chose his family over the joker but also that sense that he got what he wanted making batman strong again.