And then the handheld sequels came in. Each one featured a new method of acquiring special attacks, a new castle layout, and new characters ... and that's pretty much it. The environments tended to be similar, the weapons, though accessed in a different manner, started to run together, and it became increasingly obvious that many of the enemy sprites were being reused. We started seeing the same content, or minor variations of that content, over and over. In short, the longer the current style of games runs, the more likely it is to drop precipitously into staleness, if you don't already think it is stale.
Another issue that has popped up over time is the room design. The castle layouts have all been very cool, but the individual rooms have sort of stagnated into rectangles with a few flat, floating platforms. This is mostly out of necessity due to the connected nature of the castle -- the rooms themselves aren't that complicated, because navigating the castle as a whole is. But the level designers have gone too far in the "boring" direction. The crazy gravity-changing rooms in Portrait of Ruin are a notable exception. What happened to stairs? Walking up stairs was once half the game.
There is a solution to the potential staleness in the least likely of places: the old Castlevania. While the "new" formula was a shot in the arm for the series back on the PlayStation, I don't think that anyone was actually tired of the old formula. Super Castlevania IV, Dracula X, and (especially?) Castlevania: Bloodlines are still great to play, and there's really nothing like them on modern consoles. The emphasis has been moved from action and reflexes to exploration. At this point, even a return to original-style linear level structures and difficulty would feel fresh. The rooms and levels could even go in any direction, with no need to come up with a floor plan.
The other solution is a new style of game. Yes, that often results in things like Castlevania 64 or Lament of Innocence, but those changes are too drastic. There must be other kinds of side-scrolling action games out there for IGA to experiment with. Why not a fully grappling-based Castlevania? Or a run-and-gun set in the future?