Are you using “ride system” to mean general ride movement concept? A ride system is a specific type of ride mechanism, not a general category describing experience. Only the notion of a drop ride was sort of there from the beginning, but it was because the creative teams were exploring the possibilities of a recently developed ride type.
The Twilight Zone Tower of Terror uses a custom designed and built ride system developed exclusively for the attraction. Its history goes back to explorations of drop ride concepts that started from the first generation of Intamin drop rides. A widely published example is art from the early development of Discovery Mountain at Disneyland Paris from when it was intended to be a multi-attraction pavilion that clearly show the Intamin ride system. These rides using that system were never built and instead Disney went on to develop their own, more advanced, system that better met the story that was being developed for the attraction experience that become the Twilight Zone Tower of Terror.
The attraction also has roots in the concept work done for the “Hotel Mel” attraction that was developed in collaboration with Mel Brooks. This project is the origin of the concept of the haunted hotel at the end of Sunset Blvd. This attraction was not a drop ride, but a more traditional dark ride with guests riding around a horror movie set in wire guided golf carts. That though never coalesced into a coherent idea and other ideas ended up being explored for the haunted hotel concept. One of these was a mostly walkthrough experience about actors who disappeared in the hotel with the finale taking place in an elevator that was also not a drop ride.