This is actually quite interesting, as it might be the first time in many years in F1 that seemingly no-one have a really good guess on who it'll be.
- Sebastien Buemi - while in RB already, it's apparently not something Toyota is interested in, because of the TR-Honda deal.
- Alexander Rossi - Have denied it
- Jolyon Palmer - Seems utterly unlikely from TR's PoV
- Sergey Sirotkin - Would've been more logical if TR was continuing with Renault engines for 2018
Most likely TR will be looking at a rookie to get in, it was 38 drivers pre 2017 season that had enough points to enter F1. Ocon, Vandoorne, Stroll and Palmer was in F1. While Piquet Jr, Rossi, Montoya, Davidson, Webber, Nakajima, Di Grassi, Lotterer, Sarrazin and Buemi already have F1 race(s) to their name.
Then we can remove Pierre Gasly, Sirotkin, Giovanazzi, Oliver Rowland, Louis Deletraz, Charles Leclerc and Nyck De Vries from the list as well, as they are with other jr.teams (or as in Gasly's case, is the reason why they need a "random" driver).
So, from a pool of rookies, that before the season started had enough points for a Super License, and that are not connected to another team at the moment.
- Oliver Jarvis
- Tom Dillmann
- Felix Rosenqvist
- Jose Maria Lopez
- Alex Lynn
- Simon Pagenaud
- Timo Bernhard
- Brendon Hartley
- Helio Castroneves
- Loic Duval
- Scott Dixon
- Benoit Treluyer
- Marcel Fässler
- Romain Dumas
- Marc Lieb
- Neel Jani
- Will Power
In addition to this Josef Newgarden won IndyCar in 2017, and will have 40 points. As the season is over, I am fairly sure they can make a case to get him in the car based on that.
I would narrow it down to a list of 4 possible drivers, and ranking them in order of probability in my mind.
- Brendon Hartley - Still only 27 years, is sponsored by Red Bull on a personal level, so while not in the official Red Bull - gang of drivers, he have connection to RB. Previous test/reserve for RB and also done young drivers test for Merc. He is however under contract with Porsche still, and are one of the front-runners for the second Chip Ganassi seat in IndyCar next year.
- Sebastien Buemi - Much experience, well known in RB, been the reserve for RB since he got out of TR. However, Toyota isn't too happy about it per the reports. Also, TR seemingly are looking for a potential 2018 driver from it as well, and it's been clearly stated that Buemi is not an option for 2018.
- Neel Jani/Josef Newgarden - Jani is, together with Hartley still sponsored by Red Bull on a personal level. He is 33, and it's in these days to late for a F1 career debut, but Hill was 33 when he got his Williams-seat after a half season with DNQ's for Brabham in 92. If TR are unable to wriggle Hartley out, and Toyota refuses, Jani might have a shot.
The same goes for Josef Newgarden. Rumours have it that Liberty Media is interested in pushing Newgarden in, and from tradition, I don't think the US-teams (Penske in this occasion) will stand in the way for it. However, I don't think Newgarden have anything to gain from jumping in to cars he's never been in, to race on a new circuit with zero testing. But PR-wise, I think Liberty Media would like it.
So... that's my guess.