I have an idea for a weapon that can permanently absorb items that require attunement to gain its properties (including its attunement requirement), up to 3.
For example, the item alone does not require attunement.
It then absorbs a pair of Boots of Speed. We will ignore the requirement of "click your boots together", and instead the user must take a bonus action to perform an appropriate task (perhaps tapping the weapon on your legs) to activate the item's Boots of Speed property, which gives the normal benefits of the boots of speed. The item now requires one attunement "slot" (of 3).
Further attunement items absorbed into the initial item gain their properties and raise the attunement requirement, up to 3.
Because artifacts are of nebulous power, and therefore have not many guidelines, the following question pertains to only the rarities common though legendary.
Is this property inherently imbalanced? If not, what would the rarity be of an otherwise mundane weapon with this property?
Best Answer
Since this item does not let you evade or increase your Attunement Slot limit, the main way in which it increases your power is that it would allow you to wear multiple items that go in the same body "slot". The Magic Items section of the Players Handbook says:
With this item, it would be possible, in theory, to gain the benefits of multiple boots or hats or cloaks at the same time. It's hard to say how powerful this is, as it depends on the specific magic items to be combined. I am sure that there are extremely powerful combos that would be enabled by this, but I don't think it's inherently unbalanced.
You might want to prevent this weapon from absorbing powers from other weapons. More than other items, weapon abilities are balanced around the idea that you'd only benefit from 1 (or 2) at once. Combining a Dwarven Thrower, a Flame Tongue Sword, and a Holy Avenger, for example, would give you a +6 weapon that does +1d8 +2d6 +2d10 (vs undead) bonus damage on each hit.
You might also want to address whether identical bonuses stack. For example, if you combined 3 copies of the Cloak Of Protection, you would add +3 to AC and all saves. I wouldn't say that's overpowered (since it uses all 3 of your attunement slots) but you could imagine that combining more powerful items could generate very large bonuses that break bounded accuracy.