The Hawaiian plate lunch sandwich takes a meal that is defined by not being a sandwich and folds it into bread anyway. The plate lunch is a fixed Hawaiian format: a scoop of macaroni salad, two scoops of white rice, and a protein, served flat on a partitioned plate. Turning it into a sandwich means committing to the thing most American sandwiches are built to avoid, which is starch stacked on starch. Rice and macaroni salad are the filling, not a side, and the protein shares the build with them rather than dominating it. That deliberate carbohydrate density is not a flaw to be engineered around. It is the entire identity, carried over intact from the plate it came from.
The craft is in making that density hold together and read as balanced rather than heavy. The macaroni salad is the binder by design: a soft, mayonnaise-heavy, slightly sweet salad that glues the rice and protein into a cohesive mass instead of a pile that spills. The protein carries the salt and savor the starches lack, whether it is teriyaki beef, kalua pork, fried chicken, or a slab of griddled Spam, and it is sauced or seasoned strongly precisely because it has to season two bland components at once. The bread is a soft, sweet roll that yields rather than fights, chosen because a crusty loaf would turn a soft, heavy filling into a wrestling match. Acid is the missing note the build has to supply, often through the protein's marinade or a sharp pickle, since without it the sandwich reads as one long soft sweet thing. It works for the same reason the plate lunch works: it is a full, cheap, filling meal eaten one-handed, and the design never apologizes for what it is.
The variations track the plate lunch's own roster of proteins rather than changing the structure. The teriyaki beef build, the kalua pork build, the chicken katsu build, and the loco moco read folded into bread instead of plated all keep the rice-and-mac frame and swap the center. Each of those belongs to the dense long tail of regional specialties built around a decision a national chain would never make, and each deserves its own article rather than being crowded in here.