I think the DDO developers have done an excellent job of making Quivering Punch a useful ability with the Update 19 changes that allow shattering items to increase the DC, and I think it's time to give Assassinate the same treatment. My Assassin is retired right now, but my half-assed calculations tell me that a DC of 60 is around what an intelligence based Assassin can achieve without anything silly like a yugo pot or a +2 to all stats bard song.
If my calculations are completely off and your Assassin is able to get a low 70s Assassinate DC, then please let me know so I can revise or remove this thread. Otherwise, I think a 70+ DC is what should be attainable for an int based, full sneak mode, pure level 20 Rogue Assassin.
Getting a tactics bonus to Assassinate like Quivering Punch got would probably make the DC too high since Assassinate has a higher starting point than Quivering Punch, but it needs a boost from somewhere in my opinion.