I have substituted tuna for salmon in baked salmon patties and they turned out well. One does have to up the seasoning because tuna is quite bland. I would think you could do the same sort of thing with mackerel. We like ours with homemade guac or tartar sauce.
And there's always old fashioned salmon pie and salmon and corn pie. I've used mackerel in that. Kids were not fans but I thought it tasted ok.