It's not that hard to swap the lantern for another item. It is however pretty difficult to change the animations of the player hand.
If you want to make a new item (that will replace the lantern) you'll have to open the model editor and import the mesh you want (wine bottle, hammer, etc). Next, change the User Defined Variables to HandObject.
Save your item in the "amnesia the dark descent\models\hand_objects\lantern" folder. It's best to name it something like "My_Hand_Object", that way you wont accidentally save over the original items.
Find the "lantern.ho" file (in the same folder), copy it and change the name to "lantern_BACKUP.ho". Open the original lantern.ho file and change "hand_lantern.ent" to "My_Hand_Object.ent".
Run the game, press F (make sure you have the lantern in your inventory) and enjoy your new item.
*Few more things*
If you want to make a light source (lantern, glowstick, flashlight), open your item in the model editor and add a point or spotlight.
If you want to change the lantern sounds, you'll have to change the game.cfg file (amnesia the dark descent\config\).
To remove the swinging lantern physics, open the lantern.ho and change HasSwayPhysics to "false".
Take a look a the
flashlight mod and see how those guys did it.