I think their pattern is pretty well established and their team makeup is also. I'm not sure how the ideas will come at us but if we get more supernaturals they will be put in GP's (I'm pretty sure). If we get more vacation like destinations they will be in GP form also. Low-balling how many that is I'll guess at 4.. GP wise that's 2 years worth there. I do think they will explore other ideas in GP's also on top of that.
I'm assuming we will get a University or a school based EP and yes some sort of Showtime/Superstar type of thing in an EP. I think we'll get more than that also. I think enough to keep the Sims 4 going for at least 3 more years. I'm personally hoping 4-5 more years to explore new ideas. I don't know how Seasons will come or what it will be paired with. If it's only weather it could come in a GP or patch. If it's farming attached or attached to activities that would put it in an EP.
There might not be a peep about a Sims 5 because the run is expected to last awhile. It seems like it's selling well... They might also be following another formula where they complete one before starting on another.. or at least keep it under wraps longer.
