Which rarity level is appropriate for this nerfed Scimitar of Speed with no bonus to attack and damage rolls?

Question

The original Scimitar of Speed is among the Magic Weapons listed in the DMG:

Scimitar of Speed

requires attunement, very rare

• You gain a +2 bonus to attack and damage rolls made with this magic weapon.
• In addition, you can make one attack with it as a bonus action on each of your turns.

Being able to use your bonus action for a melee weapon attack regardless of any other actions on that turn can be very useful for melee casters. Hence, I see the second bullet point as the main feature of this weapon, while the bonus to attack and damage rolls is nice but nothing unique.

The following weaker variant trades the bonus to attack and damage rolls for a lower rarity level.

Scimitar of Speed (weaker variant)

requires attunement, rare

• You can make one attack with this magic weapon as a bonus action on each of your turns.

Is this variant balanced? If not, what would be the appropriate rarity level to fix this?

Answer 1

Probably Rare

There are examples of weapons with no modifiers to attacks/damage but that offer other bonuses like the Weapon of Warning (DMG, 213).

With the release of the Eberron: Rising from the Last War, there is also a weapon available that offers mundane damage but provides an extra bonus action attack: the double-bladed scimitar.

So what you've got with your homebrew is this mundane weapon, but have made it magical for the purpose of bypassing immunities/resistances of mundane damage. That's basically working the same way as the Weapon of Warning and that seems like the best comparison.

This aspect alone seems to suggest that it's simply uncommon, but we now need to consider that you get an always-on proficient bonus-action attack.

Bonus Action Attacks

You typically need to invest in a fighting style or feat in order to make a proficient bonus action attack - and even those still require a main-hand attack first. Because of the always-on availability of another attack that this gives you, I think I'd rate this as a Rare weapon.

You can also compare this to a vicious weapon, which also doesn't provide any bonuses to hit/damage, but adds 7 damage to the attack when a natural 20 is rolled. That's approximately the same damage total for 1d6+Modifier of the scimitar, but the trade-off is instead of requiring it on a natural 20, you get another attack roll.