I've noticed in some recent reading. . . .
One thing that makes expanding a series indefinitely hard -- even if you switch view point characters to make character development plausible -- is that it's hard to introduce new elements without breaking the unity.
How different the element is matters, of course. It's easier to introduce dragons into worlds that already have unicorns and gryphons than those that have none; a variety of fantasy or ET races allow you, usually, to introduce a new one -- though it helps if it's the same sort of ilk.
But too many alien elements break the unity.