Algarotti/Heinrich is nominated because it's the crack headcanon of mine that totally works in combination with other, more serious pairings as well as on its own. I mean: Algarotti/Heinrich one night stand might be a fun short story on its own, or it could work in combination with Fritz/Algarotti as a side issue of Algarotti venting his Fritz frustrations or preparing his Frexit or what not. (While Heinrich is totally game for this because a) Algarotti is a sexpot, and b) it'll annoy Big Brother. Also, depending on the timing of the one night stand, c) payback for Marwitz, or payback for the marriage enforcement.)

Fritz/Voltaire: I'm actually going to dare one of you to write an AU where Fritz is the ambitious ficathon organizer who finally managed to woo BNF Voltaire for his ficathon (and as beta to his own fanfic), and then Voltaire starters to get into it with other BNF Maupertuis and with Fritz himself because someone forwards Voltaire's secret emails about laundry washing to Fritz and things escalate from there. :)