This topic has been
had many times before. Basically it's very unlikely there will be a third one, as going by FR/LG they remained a duo, and far more significantly HG/SS has the main features that Crystal had, which was the Suicune/Eusine sideplot, a Battle Tower (only now there's a Battle Frontier), expansions on the Ruins of Alph... there's very little from Crystal not in HG/SS now, which boils down to the Odd Egg and the lack of Krys (and there's Kotone as the female protagonist anyways). There's just little point.
Closed, because discussion has been done many times already.