Customizable Sandwiches

Crafting

You could put a piece of bread and then any other food items in a crafting table to make a Sandwich.

Functionality

The Sandwich's restored hunger and saturation could be equal to the combined hunger and saturation restoration of the used food items (including the piece of bread) plus a couple extra points to make the sandwich actually worth crafting.

You could put a Sandwich in a crafting table alongside more ingredients to add even more of them into the sandwich.

Balancing

To not make this super OP and allow people to make mega-sandwiches that restore a full hunger bar in 1 second, they could take slightly longer to eat the more ingredients there are in the sandwich.

Appearance

Its item texture could also be auto-generated from the ingredient items' textures - taking a bottom bread slice texture, putting those food items' textures in the middle, then putting a top bread slice texture on top. (it could even extend beyond the 16x16 limit if enough ingredients are used, much like the spear when held, for comedic effect)

Special Item Uses In Sandwiches

Some food items could be restricted from being added to sandwiches, like cakes and pumpkin pies, or have special textures when in a sandwich, like milk (taking the appearance of a cheese slice).