red sled,
When substituting one fish for another I consider 2 main qualities, texture and flavor. I then consider each quality in 3 classifications. With texture, it's delicate, moderate, or firm. With flavor, it's mild, moderate, or full. Using that methodology, I consider halibut a firm and mild fish.
Though I can't think of a fish that would be a direct substitute for halibut, grouper, snapper, walleye, and turbot would be fine alternatives. Also, I wouldn't consider cod a substitute as while it has a mild flavor, the texture is very delicate.