Here's what I think a reasonably-feasible grid would look like:
Red Bull - Sebastian Vettel and Jean-Eric Vergne
Vettel remains at Red Bull, confident that Adrian Newey can continue to deliver high-quality cars. Webber, however, decides to retire on a high note, and leaves the team. Although Vergne proves to be a weak qualifier, he is the better racer of the Toro Rosso pair, and the team feel that with Vettel in the first seat, they can afford to take a little time to shape Vergne into a better racer.
McLaren - Jenson Button and Lewis Hamilton
Despite constant rumours that Hamilton will move to Red Bull as soon as it becomes apparent that Webber will leave, both he and Button stay at McLaren, who are banking on consistency in their driver line-up giving them an edge for 2013.
Ferrari - Fernando Alonso and Sergio Perez
In a move that surprises no-one, Felipe Massa is shown the door at Ferrari. Despite almost every single driver (except Narain Karthikeyan) being mentioned as a potential replacement, Sergio Perez is the team's first, last and obvious choice for the second seat.
Mercedes - Nico Rosberg and Paul di Resta
Michael Schumacher decides to re-retire at the end of the season following protracted debate over the Pirelli tyres. Mercedes look to Force India for his replacement, and settle on di Resta as he is consistently a stronger performer than Hulkenberg.
Lotus - Kimi Raikkonen and Romain Grosjean
Perhaps the hardest team to call, simply because so much hinges on the outcome of the Group Lotus sale. China Youngman has openly displayed interest in the company, and if they buy it, they might be interested in Formula 1, in which case they might apply pressure for a Chinese driver. Or they might sever all ties to the team and request that they use a new name. Or Gerard Lopez and Genii Capital could buy the company. it all depends on who comes out as the new owner of Lotus Cars.
Force India - Nico Hulkenberg and Robert Wickens
Hulkenberg fails to impress in the middle of a tightly-fought midfield, and only saves his seat when di Resta is called up to Mercedes. The void is filled by the highly-impressive Robert Wickens, who despite a poor DTM season, still shows plenty of promise.
Sauber - Vitaly Petrov an Esteban Gutierrez
With Perez off to Ferrari, Gutierrez is his natural successor. Petrov's placement is a little harder to define: the team has accepted money from Chelsea FC, owned by Russian expatriate Roman Abramovich. Foreshadowing a potential buy-in (that would secure the future of the team for the next decade), Abramovich uses his influence (and mandate from Vladimir Putin) to place Petrov at the team. It is not difficult to justify cutting Kobayashi loose after a string of disappointing finishes; it has, to date, been a year since he scored points in three races in a row in a single season.
Toro Rosso - Daniel Ricciardo and Kevin Korjus
Both Vergne and Ricciardo underperform in 2012, creating an interesting situation at Red Bull. Despite rumours that both Hamilton and Raikkonen will join them (when asked if he would move, Raikkonen says "no", which his fans interpret as "yes"), they take Vergne as they do not want someone entering the team and upsetting the order. Ricciardo's team-mate is Formula Renault 3.5 stalwart Kevin Korjus.
Williams - Valterri Bottas and Kamui Kobayashi
The FW34's performance brings a renewed confidence to Williams. Bottas replaces Senna mid-season, and a string of good results means the team is no longer dependent on pay drivers. Sir Frank decides to take a chance, cutting Maldonado loose in favour of a more-traditional Williams driver - Kamui Kobayashi. His signing is heralded as a perfect match of team and driver.
Caterham - Hekki Kovalainen and Alexander Rossi
With Petrov moving to greener (more like white-er and grey-er) pastures, the team happily take Alexander Rossi on. Kovalainen stays with the team, edging closer to Q2 with each race.
??? - Dani Clos and Pastor Maldonado
Kathikeyan and de la Rosa are dropped in favouf of Clos and Maldonado. Clos brings the promised Spanish flavour, while Maldonado and PDVSA fund the team.
Marussia - Timo Glock and Rio Haryanto
Marussia secure 10th place in the World Constructors' Championship through sheer reliability. Glock flirts with the idea of retirement, but the team's long-term plans combined with a steady stream of Russian petrodollars and the placing in the WCC convince him that he made the right choice when he joined in 2010. Charles Pic is dropped in favour of Haryanto, who impresses in a much-improved Carlin in GP2, bringing Indonesian money to the team.
