The paper specifies a couple of restrictive characteristics for some of their models. I would be curious how things would change if these restrictions were weakened or removed. For example, the paper presents a HMMs that have only four types of states and in a specific configuration. Although this seems like an intuitive system/configuration, I would be curious to see the performance of other document models. My intuition is that the model defined is fairly robust, however, some complex documents, fields or domains may require a more complex model.