In an interview with Stellan Skarsgård it is revealed by the actor that discussions have taken place, the studio definitely like the idea and it is now down to the creative team ("the three girls") to come up with an idea that everybody likes.
Remember folks, that rumours about a Mamma Mia! 2 will swirl around right up until if/when one is announced and it’s definitely best to take them all with a grain of salt. If, as Stellan mentions, discussions have taken place, then they will be at a very early stage of the process, for sure!
My thanks to Niclas for news of the article.