According to Marchionne, if the new Quattroporte (to be launched at Detroit) sells well, a smaller model will be put into production: the Ghibli. This car's architecture will be shared with Alfa Romeo.
As it is, Maserati and Alfa will share development of a V6 petrol engine to be used in Giulia and a possible "4C GTA" ("6C"?), and even if the Ghibli doesn't get the green light, this engine is a possibility as an entry engine for the QP: Marchionne has repeatedly said he wants Maserati to have a wider audience - adding a 3.x litre petrol producing about 300-400bhp would provide a good "entry" model in the QP range without cheapening the brand.