How about this sandwich? That would have been an interesting development meeting to be part of. This really is a cross between the offerings of both chains, with a pastry like bun and spicy filling. Here are the layers in order: French cruller bun, a spiral-shaped sausage, hot chilli sauce, onion, tomato, lettuce, mayonnaise, lower French cruller bun.
I went by and tried this and have to say it is actually pretty good, much, much better than I thought it would be. The chilli is spicy, but nevertheless goes fine with the cruller bun. Since it needs a little more texture to not fall apart, the cruller reminds me more of a waffle than a delicate pastry. モス ミスタードーナツ フレンチクルーラー
Photo is a web capture for explanatory purposes, copyright belongs to the company.