First, I’d like to say that I love any and all ideas that make technology more available and approachable to non-experts. So, my direct answer to your question of “will this be interesting to designers?” is an emphatic, “YES!”
However, this is not a new idea. There are tons of systems/products on the market that provide a way for non-experts to add computer controlled devices to their projects. There are even levels of difficulty within this type of products, from toys aimed at children (Little Bits and Lego Mindstorms) to more complex systems aimed at amateurs (Arduino & Raspberry Pi) to higher complexity systems/software aimed at professionals (LabVIEW). If a designer dreams up some kind of product or interface that would require robotics to execute, they can turn to one of these systems to try to make their design come to life.
The key to designing something like a “very easy interface for developing robotic applications” is finding the balance between simplicity and functionality. In order to simplify the system, you are going to have to take out (or hide) some of the functionality. But by limiting the functionality, you are limiting the types of projects that it can be used for. As soon as the designer dreams up something that has more rigorous requirements, they can no longer use the simple system and have to either become the “robotics geek” by learning about the more complex hardware/software or hire one to do it for them.
For example, I am a designer & mechanical engineer, and I have done a few projects using Arduino boards to create custom interfaces. The Arduino platform is great, because as a beginner, it is very easy to go and buy a servo motor and download a sketch that controls it. Then you can go buy a rangefinding sensor and copy and paste the sketch that allows you to read in sensor data. However, now you need your servo motor to perform specific actions in reaction to the data that your rangefinder is providing, but you can’t find a sketch that does exactly what you’re looking for. Now you have to dig a bit deeper and learn more about the hardware/software to achieve your desired result. You can quickly get sucked into a black hole of internet research to try and learn enough to accomplish your goal. By the time you’re done, however, you are now effectively a robotics geek compared to your former self. I think few designers would tell you that they’re simply not interested in learning about the technology. They probably just don’t have enough time to devote to learning about the technology, so it is much easier to work with someone else who already has the knowledge.
My advice would be to think very carefully about who will be using this interface and how they might want to use it. Then think about how a beginner might use the system, and how they might want to push the boundaries of the system once they have mastered the simple functionality. Creating a system that the user can grow and learn with will be the key to whether your interface is successful or not. Good luck!