In some sports leagues (particularly in the USA), there is a contingency plan named the "disaster draft". In case a team gets involved in a horrible accident that kills or incapacitates most of its players, it can draft players from the other teams to continue participating.
While such a situation does not exactly translate to a racing series, let's imagine a different (but similar) scenario. All 20 drivers that are supposed to drive in Formula One in 2019 are killed in a plane crash. Team staff, reserve drivers, FIA/Formula One officials, and others involved in Formula One are unharmed. Despite the tragedy, all involved parties decide not to postpone any races and to continue preparations for the 2019 season.
Which drivers do you think would be on the grid in such a scenario?
Mercedes: Ocon/Bird
Ferrari: Alonso/Rossi
Red Bull: Buemi/Vergne
Renault: Rosenqvist/Rowland
Haas: Pagenaud/Nasr
McLaren: Newgarden/de Vries
Racing Point: Markelov/Wehrlein
Sauber: Ericsson/Kobayashi
Toro Rosso: Hartley/Yamamoto
Williams: Sirotkin/Massa
For some of the drivers I had a specific team in mind. Alonso would likely be the top free agent, but there are rumors Mercedes are not very fond of him – so he's likely to go for Ferrari instead. Mercedes would quickly lock down Ocon. Red Bull would be the most likely team to sign up Buemi and Vergne, two Red Bull-affiliated drivers, from Formula E. I think it would be harder for them to fill the Toro Rosso seats – Hartley might agree to return, but someone like da Costa might not want to leave a top Formula E seat for anything less than a Red Bull. Sauber and Williams would ask Ericsson and Sirotkin, respectively, to return.
Some drivers I added to the grid without such a clear idea for their teams. Sam Bird, Alexander Rossi, Josef Newgarden, and Felix Rosenqvist are top performers in Formula E and IndyCar, and would likely be approached by multiple teams in such a situation. None of them have been affiliated with any of the current teams to my knowledge (except Bird, who tested for Mercedes in 2010), but all of them are young and talented.
Then I filled the blanks with drivers that either have some connection to their team or engine supplier (Rowland/Renault, de Vries/McLaren, Kobayashi/Sauber, Yamamoto/Honda, Massa/Williams), recent F1 experience (Wehrlein, Nasr), general experience (Pagenaud) or general competence, potentially along with money (Markelov).
I tried to only use drivers that have qualified for a FIA Super Licence (although I believe some might lack a point or two), but it's likely the FIA would not enforce the points requirement so strictly in such a situation.