Along the line with mushrooms.....spread mayo on the top bun put the hot/sauteed mushrooms and throw swiss ontop of that; combine and eat to mucho goodness.
someone once told me they put crushed bread crumbs in the meat, it ended up being really good, but then again i was drunk
This was actually going to be my suggestion. Breadcrumbs, a small amount of egg yolk, salt, pepper, garlic. It is somewhat similiar to meatloaf on a bun, but hell I ain't complaining. I love meatloaf.