If you keep on doing the same old thing in the series, you bore the reader. If you keep on introducing new things, after a time the readers wonder why they never heard of them before.
Some series manage better than others. A murder mystery series can introduce new murders without getting too monotonous. Doctor Who, since it was jaunting through all of space and time, could land anywhere.
But I remember a series where the hero was a hard-bitten veteran of the world, and always was familiar with the monsters and even intelligent species that were introduced in book seven or eight. Nothing says the world is unreal like introducing entire kingdoms of beings that weren't even hinted at earlier.
It can be very difficult to hint at things without actually bringing them into the story (and derailing it), but series should end before it becomes clear that the writer is tacking something on to create a new story.