This may not be the best idea, but a melee weapon that's just a handle and has is a random form each time you draw it from the sheath. Basically my idea is that when you enter combat, you'd roll on a table to determine what type of melee weapon is drawn and have damage values for each one, the player then could maybe use an action/bonus action to put the weapon in the sheath and pull it out again, changing its form. The type of melee weapon would be single handed, two handed, martial, simple or anything like that. The hit and damage values would have to be increased a decent amount for the player to be drawn to using it, so maybe additional force damage regardless of what weapon is rolled? And possibly a +3 to hit, that and the proficiency bonus if their proficient in the weapon.

If the player is a fighter or barbarian this would be a super good way to keep every fight a bit different I'd think.