BMW, if they were to return, would have to do so in 2017. Mercedes have 4 teams in 2014 with Mercedes AMG, Force India, McLaren and new addition Williams. McLaren will leave at the end of 2014 for Honda. Ferrari have their Factory Scuderia team, but will keep Sauber if they sort their financial problems and add Marussia, both teams taking the whole Scuderia transmission as well as Energy Recovery System. Renault will have Red Bull and for the first time sister team Toro Rosso, Renault will likely pick up Lotus as well.
As for the contracts, Force India and Williams are a 3 year contract, McLaren a single year one with Mercedes before Honda join on a probable 8+ year contract. Red Bull are the “works” Renault team and Toro Rosso in for the same time as their sister team. Lotus look to be taking Renault on for a 3 year deal, but probably offset on a reduction in price in order for Renault to take out a key sponsorship area on the cars as Red Bull will have Infiniti plastered across their cars and to the man on the street makes Red Bull a Infiniti powered car. Ferrari have a 3 year deal with Marussia, Sauber will have a probable deal of same or similar terms. Both Ferrari customer teams will have to take a Ferrari Academy Driver if Ferrari want them to, this means Bianchi progressing through the field at Ferraris beck and call.
As for a BMW return, a Toyota return even. All highly doubtful, the regulations are in place to entice manufacturers back in as a engine supplier yes. However I think it is all a lot of ifs and buts. If anything, Honda are likely to pick up a second customer in 2016 by either a 12th team or another team of the other 10 excluding Mercedes, Red Bull or Ferrari breaching their contract and gaining Honda. If BMW are to return they will have to buy a team and breach a contract to put their product in the back of their car. When new engine suppliers are looking to spend €120m a year on the V6T engines, with as much as half of that coming back in engine supply contracts, I doubt BMW will come back to spend €200+m a year in a Team and Engine side of things, even if they get €30m back in engine supply contracts.
BMW are happy at present with their DTM and ALMS program’s, and rumours persist they are looking at the viability of a MotoGP campaign in 2015 as well. 3 areas of the economy where BMW are doing well, showing off their engines in what is effectively 2 spec series in DTM and ALMS as the DTM chassis is developed by Dallara with a body shell developed by the manufacturer. Motorbikes are doing good for BMW, not Ducatti, Honda or Yamaha well, but are doing well enough for them to look at breaking into MotoGP.
As with the recent Vodaphone/Verizon deal in the USA, monopolies ebb and flow in business, engine suppliers do the same in F1. When the time is right, and when the economy is viable, BMW and Toyota will return.