Hangul is a beautiful constructed script, used to write the natural languages of Korean, Jeju, Hokkien, and Cia-Cia, perhaps others. The shape of the letters mimic the shape of the mouth, and a letter for a voiceless consonant is directly related to the letter for its voiced counterpart. Personally, I think it's brilliant and should be adopted worldwide, perhaps with some additions to consider sounds that do not exist in Korean (by far the most dominant language using it).
One idea of constructed international auxiliary languages is that those languages should have a simple and logical grammar to be easy to learn. But it appears most of those languages are still written with the Latin alphabet, which doesn't have much or any connection between the look of a letter and its pronunciation — this still needs to be learned by heart. A learner cannot derive the pronunciation of a previously unknown letter from its shape.
Have there been any designs of constructed languages, aiming to be international auxiliary languages (thus not counting fictional alien/fantasy languages), which from the start had a dedicated constructed script designed to be as simple as possible, yet universally applicable?