I know some people have mentioned saving the big guys for the movies but there's already a precedent of introducing a character on Disney Plus before moving into the movie side of things. The chick in Wandavision is going to be in The Marvels along with Ms. Marvel who is also being introduced in her own show first. So I don't see why they wouldn't introduce a major villain on a show first.
For my part, I have said that I don’t think big guys would be introduced at the end to move/resolve the plot - introducing them in a mid credit so you don’t have to do 10 min of backstory in your next movie makes lots of sense