What's a good marinade for a T-bone steak?

Davetv1 is right. The quickest way to find out if someone has no clue about how to cook tender beef cuts is to let them give you marinades for it. This is one difference between real cooks/chefs and the copy-and-paste pretenders. Copy-and-paste doesn't require any cooking knowledge or skill. Most marinades make tender beef tougher, particularly if they contain any acidic ingredients. Acid doesn't tenderize tender cuts of beef: it breaks down the cell walls and makes it more tough, dry, and chalky after cooking. Try the following experiment. Take a filet mignon steak, cut it in half, place each half in a ziplock bag, one with 1 tsp of kosher salt, the other with 2 Tbsp of vinegar. Let soak for 48 hours (for full effect). Then cook them side by side to medium, exactly the same way. Then discover for yourself what the acid does to it.