This item appears as a short sword which contains an elaborate, partly visible clockwork mechanism in the grip.
As a bonus action the wielder can activate the items mechanism and transform it into either a longsword or a great sword. It can be transformed back into a short sword in the same way. The item changes its shape and weight to match the desired form and functions like a regular weapon of that type.
In all forms the sword is magical and the wielder gains a +1 bonus to attack and damage rolls made with the sword.
Having to use a bonus action to transform the sword can be a handicap, especially in the first round of combat. E.g. the barbarian can either enter rage or transform the sword. This might keep the player from carrying around the sword in convenient short sword form, which might make the item less cool to own.
Making the transformation free or part of an attack action doesn't really make the item more powerful, and might be the better choice.
This item appears a small cylinder, about the size of a human thumb. It glows faintly with a blueish light and is warm to the touch.
The mana cell can be used to power a Gnomish Mechanical Sword by inserting it into the grip of the weapon. Inserting or changing the mana cell of a sword takes about 1 minute.
The mana cell contains a single charge which it regains daily at dawn. When its charge is expended is glows much weaker, so it is easy to tell a charged and an empty mana cell apart.
This is essentially an upgrade for the Gnomish Mechanical Sword allowing it to stay useful as the owner levels up, especially if he is fond of the weapon.
One of the ways of acquiring a mana cell that fits nicely with the theme of the weapon it is used in, is to have it power some mechanical creature of obvious (or not so obvious) gnomish origin. When the players defeat the creature they notice one (or more) more charged mana cells in its tangled remains. Additional foes of similar type might yield additional cells (each representing one use per day).
This is essentially a Gnomish Mechanical Sword with a Mana Cell inserted into its grip. All the rules of the original weapon still apply.
When an attack with this weapon is a hit, the wielder can choose to expend the charge of the mana cell. When the sword is activated in this way, hundreds of sharp metal teeth spring from the sword's edge and start to move quickly back and forth along the edge. This deals an additional 1d20 damage slashing to the target (this is also magical damage).
Inserting or changing the mana cell inside the grip takes about 1 minute.
Dealing damage in this way (1d20) is a really all-or-nothing approach, having a high chance to deal very little damage, with an ability that can be only used once per fight.
Having the damage be 3d6, 4d6 or 3d8 might yield more consistent results. However using 1d20 can seem a bit more "gnomish". Having the sword do something crazy or awesome on a 1 or 20 on the damage roll can reinforce this "gnomishness".
This item appears a small cylinder, about the size of a human thumb. It glows intensely with a redish light and is hot to the touch.
The mana cell can be used to power a Gnomish Mechanical Sword by inserting it into the grip of the weapon. Inserting or changing the mana cell of a sword takes about 1 minute.
The mana cell contains 5 charges which it regains daily at dawn. The more charges are expended the fainter the glow gets.
This is essentially another upgrade for the Powered Gnomish Mechanical Sword.
This is essentially a Gnomish Mechanical Sword with a Infernium Mana Cell inserted into its grip. All the rules of the original weapon still apply, however the bonus to attack and damage increases to +2.
Just like the Powered Gnomish Mechanical Sword it can be activated on a hit to deal extra (1d20) damage. But is has the advantage of having up to 5 charges.
Inserting or changing the mana cell inside the grip takes about 1 minute.
It is also possible to make the Infernium Mana Cell non-removable, so that it permanently transforms the sword it is inserted into. Since the sword now has 5 charges, it will rarely be necessary to exchange mana cells (if the player has more than one available).
Making it possible to use both types of mana-cells adds further complexity, as the attack/damage bonus could change (although you can just say, the bonus doesn't reduce any more, once a Infernium Mana Cell was inserted once).
A magical longbow that grants a bonus of +2 to attack and damage. Then range property is increased with a short range of 300 feet and a long range of 1200 feet.
The bow has 10 charges. While holding it you can use a bonus action to expend 1 or more of its charges to cast one of the following spells from it, using you spell save DC: hunter's mark (1 charge), ensnaring strike (2 charges), or lightning arrow (4 charges).
The bow regains 1d6 + 4 expended charges daily at dawn. If you expend the last charge, roll a d20. On a 1, the item becomes a nonmagical longbow.
The spells on this item are all from the ranger spell list. So you might want to require attunement by a ranger.
The "last charge risk" was copied from similar items in the Dungeon Masters Guide, but it can lead to the players using the item too cautiously. On the other hand this can add some suspense when this applies in a dramatic moment.
A pair of magical short swords that grant a bonus of +2 to attack and damage.
When you are wielding both swords at the same time, you gain the benefit of the two-weapon fighting style which allows you to add your ability modifier to the damage of the attack granted by two-weapon fighting (Player's Handbook, page 195).
This pair of items only counts as one item for attuning to them. Furthermore if you only possess one of the swords, they grant no benefit and act as magical but otherwise ordinary short swords. No attunement is possible when possessing only one of the swords, but attunement is not broken when you loose only one of them.
When they are separated the swords grant their owner the ability to know the exact location of the other one if it is on the same plane of existence, or which plane of existence the other one is on otherwise.
Bepher and Nepher can be any other one-handed weapon that has the light property, if this is more appropriate for your setting or your players e.g. hand axes.
If you want to make this item more powerful, you could also let it grant the benefit of the the dual wielder feat (Player's Handbook, page 165). This would also allow these twin weapons to be any one-handed weapons e.g. battle axes, not just the ones with the light property.