It's several months already since I think a lot about Franz Joseph being the only one who der Tod addresses with Sie (formal you).
Yes, maybe it's not a quite relevant observation since we have only two more people who Tod directly communicates with (Elisabeth and Rudolf; I don't mention Lucheni 'cause the only phrase told to him doesn't help to understand if Tod uses du or Sie; and we're not counting Maladie where he pretends to be the doctor because... well, he pretends to be the doctor).
I like the idea (headcanon?) that Tod addresses everyone with informal du because everyone is equal to death. That's why if we assume that Franz Joseph really is the only one Tod decides to addresse with Sie, it gives space to many interesting interpretations. I can name at least two:
1. Tod actually respects Franz Joseph (either as an emperor or as a person; more likely as a person, but who knows). It's the least possible variant, I think, if we're taking the context of the musical in account, but I've decided to mention this possibility. It's an interesting turn of events. And who knows what's going on in the head of this inhuman being anyway.
2. Tod is just mocking Franz Joseph by choosing more formal pronouns. Something like "Well, I will address you formally, but you understand that it means nothing, don't you?" Because the whole their conversation is Tod showing his power in the situation. The formal address (that is supposed to show respect) stresses the contrast between the words that Tod says and the form, between the real state of things and expectations. The line "Das ist doch Ihr Albtraum" (but it's your nightmare) hits me really hard every time. Once again, because of the contrast: Franz Joseph is the emperor. He has authority and power, but it's still not enough to save his loved ones and his empire. He is powerful according to his status, but the reality shows that his power is limited. In the same way, Tod uses the formal, respectful address, but he really doesn't mean such attitude to Franz Joseph.
Maybe I'm just delusional and see hidden sence and parallels where there's none, but I like thinking about it.