ADDIE or ISD is no one thing in practice. There are almost as many iterations as there are practitioners.

Some see ISD as procedural, rigorous, characterized by one box each for analysis, design, development, implementation and evaluation, with arrows linking the boxes and dependable steps directing what to do and in what order. Others see it differently. They emphasize what goes on within the boxes, inclining towards a more heuristic approach (Clark’s checklists), with rules of thumb considered as the process moves forward. In the former, instructional design is a favorite recipe. In the latter, it’s about continuous tasting, guided by a mental model derived from the literature, data, and past successes.

Here is what gives ADDIE its addie-ness:

• There is purpose and defined process. Even though everybody does not see it the same way, most agree that data enlightens decision and output from one phase serves as input for subsequent actions and decisions.
• Theory drives practice. There are reasons for the decisions that are made, and those decisions are based on the literature and best practices regarding learning, communications, technology and culture. Today we look at web usability studies and examples to guide decisions about interfaces for online learning.
• Data direct decisions. Instructional designers make decisions based on data from many sources, including clients, job incumbents, the literature, work products, and error rates. Data focus the instructional designer’s attention, and educates the customer.
• Causes count. Once the mission is defined, instructional designers want to know why? Why are appraisal forms flawed? Is it that they don’t know how or that they don’t think it’s worth doing or that doing it results in a hassle? Why does the group in Belgium do it, when the group in Boston doesn’t? Solutions are systems and are born of information about causes and drivers.
• Instruction is good, but not sufficient. Wise instructional designers ask questions about causes in order to use instructional resources where they can do the most good. Instruction is only one thing we can do to develop and enhance performance.
• Outcomes are king. While there is disagreement from constructivists about how very royal outcomes are, most instructional designers subscribe to the importance of defining what participants will be able to do as a result of the learning experiences.

The argument that ADDIE implies a linear approach and needs to be modified to accommodate rapid prototyping seems to me a strange one (in practice, necessity may require that you do some stages in parallel or in a much shortened manner than ideal but that’s just life in the biz world). I’m just curious where in the original literature, ADDIE came to be described as a linear model…the mnemonic itself is linear but as for the theory…I bet if you could find an original article describing ADDIE, you wouldn’t see that suggestion.

So I guess I don’t see a need for a replacement just a better understanding that ADDIE is more fluid than people are saying and like any model can be integrated with other models.