History has taught us that when a Formula One world champion departs from the sport, it is best to stay away for good.

Think Nigel Mansell signing for McLaren in 1995 but having to miss the first two races as he was too fat to fit into the car. Or Michael Schumacher's seasons at Mercedes, when he was in his 40s and trying unsuccessfully to keep up with young team mate Nico Rosberg.

These were career post-scripts that neither man really needed. Even Kimi Raikkonen, who returned to F1 after taking two years out to go rallying, is better known as the man who was Vettel's compliant number two at Ferrari than as the 2007 World Champion. Still there is one former champion who feels he has unfinished business, and that's a contention that's hard to argue with.

Only two WDCs for the best driver of his era

Fernando Alonso is arguably the most talented driver the world has seen since the turn of the millennium. Give him a car, any car, and he will wring more performance out of it than seems possible. It's something that hadn't been seen in F1 since the days of Senna, and when Alonso won back-to-back titles in 2005 and 2006 to become a double world champion at just 25, a period of Schumacher-like domination seemed assured. Certainly, nobody would have believed that his title tally would remain at just two.

Getting the right drive

Yet even with almost supernatural talent, a driver needs a competitive car under him. That's why if you visit a site like MobileBet, you'll still see long betting odds on drivers like Max Verstappen and George Russell, despite them being acknowledged to be the biggest talents on the grid.

Alonso's career was blighted by ill-timed decisions. His single season at McLaren in 2007 was upstaged by the battle for team supremacy with Lewis Hamilton, and when he returned to Renault, the team was in decline, a situation spiralled to the depths of the crashgate controversy. Five years of Ferrari under achievement were to follow before the move back to McLaren. Alonso proved he could still achieve the seemingly impossible, somehow mixing it with the midfield in a car that was woefully underdeveloped. Ironically, it was after his departure that the McLaren finally started to exhibit some performance.

A 2021 comeback?

Alonso will turn 40 in 2021, but has said that a return for what will be a watershed season is a distinct possibility. He told Sky Sports that in 2021, the rule changes will address many of the problems that left him disillusioned with the sport and that the season could present "a good opportunity".

Just where that will be is hard to predict at present. According to insider rumours, Alonso's management team has made speculative enquiries with both Ferrari and Red Bull, but these have been met with a polite "No, thank you". Alonso himself is being as cagey as usual and has only said that he is "always talking to different teams".

Of course, in F1, things can change in the blink of an eye, and it is hard to see Vettel remaining at Ferrari beyond 2020 given his deteriorating relationship with the team. Anything could happen between now and 2021, but what we can say with certainty is that the rumours of an Alonso return will continue to circulate.