Sounds very much like my fish sandwich I posted a while back, I wonder if your missing ingredient was one I listed, namely cilantro.
But I do like the idea of padding out crab cakes with shrimp rather than various starch based fillers. Considering the cost of crab, I can’t remember the last time I made any such cakes, and the ones that I (very) rarely get at a restaurant seem to be at least 70% filler by volume. And I think mayo is a mistake as a binder / moisture source, it just tends to overwhelm any subtler flavors.