So, I prefer to err on the side of caution at first.
That's fine, I totally understand.
Here are the potential "curveballs" I thought of that could be thrown in. Note that some are mutually exclusive (i.e., they won't pair with one another).
* The Archbishop befriended an important city or military official on his most recent trip to the baths, and extended this person an invitation to join him at the opera on the night of the assassination attempt. As a result, security around the operahouse has been tightened slightly. The greatest security will be found in the area around the opera box, which will now feature several heavily-armed guards.
* The Archbishop's staff has received an anonymous tip that someone intends to try to harm or kill him at the opera. As a result, security has been greatly tightened around the operahouse, and everyone who enters is subjected to a search for weapons and contraband.
* The diva that the Archbishop was looking forward to hearing on the night of the assassination has suddenly come down with a sore throat, and as a result she will not be singing. Dejected, the Archbishop decides not to attend the performance, but sends a proxy in his place.
All of this information would, of course, be available to anyone who makes a Gather Information check prior to the assassination attempt. Of course, if the players blunder in (and it's starting to look like they might), they'll be landing in very
hot water. This would be a good opportunity to warn the players that they won't survive the game without foresight and careful planning.