I'm not sure about specific 'Doctor and Rose' rated M fanfiction offhand. However, you could try searching on popular fanfiction platforms like Archive of Our Own. They have a wide range of fanfictions and you can filter by the rating and the specific characters.
There are many fanfiction writers out there who create 'Doctor and Rose' stories. When it comes to rated M ones, it could involve things like more intense emotional situations or some mild language. One way to find them is to ask in relevant 'Doctor Who' fan groups on social media. There are often fans who are well - versed in different fanfictions and can point you in the right direction. Also, when you do find some, make sure to read the reviews first to ensure it's the kind of story you're looking for.