As I see it there are two possible ways there can be a positive outcome from this. I mean purely in terms of the form of the car, the fact that they are back is positive and that is not in question.
a) The 2017 rules will call for larger cars. Whatever model it is, something that is bigger than the current Yaris will be an improvement. As I recall Toyota had to get special dispensation from the FIA in order to use this current Yaris under the current rules, but I don't quite remember as it was about the time of the Wall Street crash that this thread started.
But unlikely to be anything other than a Yaris otherwise why would they have launched their return with the Yaris and said that they will return with the Yaris if it turned out to be the Auris/Corolla/GT86/Prius/Land Cruiser or whatever other options they have....
b) By the time they make their return surely this generation of Yaris will be replaced and hopefully by a much better looking, larger model. Given that it is smaller than it's main rivals and they also sell the Aygo there must be justification for the Yaris MkIV to be a much bigger car? This is the more likely outcome as no one directly involved in the WRC seems to recognise what is actually wrong with the sport - not a lot in my opinion but these shopping trolley technical regs are a big part of it!