It seems like the binding function (e.g. the hippocampus) is the next major challenge. I think this is related to what is referred to as "transfer learning", where the training a model has received in one domain can be partially applied to a new domain such that training time is reduced; in particular, "retriev(ing) whole from partial input of features" reads to me as exactly the missing link that would allow transfer learning to take place. If memory-augmented neural networks put us down that path, then I'd wager that executive control functions are less complex (at least to fake passably)...we could be in for some interesting times.