I don't think that Mercedes will pull out of DTM at the end of this year. Mercedes is the biggest promoter of DTM. If Opel get's no success than I am afraid they will quit.
Other manufacturers?
Ford - I don't understand why Ford (Germany) is not interested in DTM. Ford has factories in Germany and sells a lott of cars. I have good memories of the big Fords of the brothers Ruch (???) in the 90s.
Honda - Are concentrating on F1.
Toyota - Like Honda they are concentrating on F1. Maybe Lexus? Let's hope it. An advantage is that their motorsport headquarter (TTE) is in Cologne, Germany.
Nissan - I give this manufacturer the most chances to enter DTM. But... the reason Nissan pulled out of the German STW (super touring championship) was because it had a low promotional value.
Maybe the Japanese driver Kaneishi can make DTM more populair in Japan.
Alfa Romeo - Concentrating on ETCC and FIAT has no money for another series.
Seat - Would be nice, but choosed ETCC and Le Mans.
Renault - Why not? They are doing already F1, F. Renault 2000, F. Renault Monza, F. Renault V6, Clio Cup, Clio V6 Cup, Rally Super 1600.
BMW - Concentrating on ETCC. There is also some old pain. I have my doubts about the argument that DTM generates less exposure than ETCC. Here in Holland drivers like Albers, P. Huisman and Bleekemolen (DTM) get more media attention than D. Huisman and Coronel (ETCC).
A negative aspect of DTM is that I get always the feeling that it is Mercedes vs Audi (vs Opel) instead of drivers/teams against each other.