What can you substitute for bran in a muffin recipe?

You can use black treacle or anything dark and sweet and syrupy. It's also OK to use malt in similar quantities.

The molasses has a certain taste of its own which isn't easily copied and molasses do provide a very rich source of iron.

But most of those recipes can take anything syrupy like Apple juice concentrate in thick syrup form. Or you could use honey if you like that flavour in a bran muffin.

Also you can use Agave Syrup a naturally sweet one that.

Some people use Maple Syrup. Molasses and Treacle though are very thick and messy to handle but a muffin won't be spoiled by any of the above substitutes. You just have to make sure you don't overdo or under-do the sweetness and put up with the strong taste of molasses not being present.