If you use an artifact hammer in forging, it should have some special effects, perhaps dependent on which artifact it is. For instance, using Hammer of the Gods should always result in a blessed item.

Glass objects should be eligible for forging, since the temperatures involved in a forge are more than enough to melt and recast glass.

Also, it should be possible to forge pieces of worthless glass together to make decorative ornaments, which are worth either score or can be sold for spare money.

Some mechanic (maybe involving forges?) that consumes valuable gems to add enchantment to weapons or rings, because gems tend to become useless at a fairly early point in the game.

You should be able to use a forge to melt down gold items and turn them into gold pieces.

Mithril items cannot be forged. It’s too good of a material to apply to arbitrary items, and there is lore which would back up it not being a material an amateur is capable of smithing.

You can use a forge to combine elemental wands with weapons, destroying the wand and conferring an object property of cold, fire, or shock on the weapon.

When a thrown potion of oil lands on (or possibly even passes over) a furnace/forge, it ignites and explodes.

The chance of you using a forge to repair an item is dependent on player stats rather than being a total random crap shoot.

It’s not possible to dip an item into a forge and have nothing happen. Something should happen, if only a message, in all cases.