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For RAW, the only way to explicitly improve an existing weapon is to coat it with either:

1. Silver

   Some monsters that have immunity or resistance to nonmagical weapons are susceptible to silver weapons, so cautious adventurers invest

extra coin to plate their weapons with silver. You can silver a single weapon or ten pieces of ammunition for 100 gp. This cost represents not only the price of the silver, but the time and expertise needed to add silver to the weapon without making it less effective.

2. Adamantine. From Xanathar's Guide to Everything (page 78):

   Adamantine is an ultrahard metal found in meteorites and extraordinary

mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons. Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

Any other options would be completely up to the individual DM's discretion.

As to why there are so few RAW options to improve weapons, I believe that the answer is bounded accuracy.

For RAW, the only way to explicitly improve an existing weapon is to coat it with either:

1. Silver (Player's Handbook, page 148):

Some monsters that have immunity or resistance to nonmagical weapons are susceptible to silver weapons, so cautious adventurers invest extra coin to plate their weapons with silver. You can silver a single weapon or ten pieces of ammunition for 100 gp. This cost represents not only the price of the silver, but the time and expertise needed to add silver to the weapon without making it less effective.

2. Adamantine. From Xanathar's Guide to Everything (page 78):

Adamantine is an ultrahard metal found in meteorites and extraordinary mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons. Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

Any other options would be completely up to the individual DM's discretion.

As to why there are so few RAW options to improve weapons, I believe that the answer is bounded accuracy.

For RAW, the only way to explicitly improve an existing weapon is to coat it with either:

1. Silver

   Some monsters that have immunity or resistance to nonmagical weapons are susceptible to silver weapons, so cautious adventurers invest

extra coin to plate their weapons with silver. You can silver a single weapon or ten pieces of ammunition for 100 gp. This cost represents not only the price of the silver, but the time and expertise needed to add silver to the weapon without making it less effective.

2. Adamantine. From Xanathar's Guide to Everything (page 78):

   Adamantine is an ultrahard metal found in meteorites and extraordinary

mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons. Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

Any other options would be completely up to the individual DM's discretion.

For RAW, the only way to explicitly improve an existing weapon is to coat it with either:

1. Silver

   Some monsters that have immunity or resistance to nonmagical weapons are susceptible to silver weapons, so cautious adventurers invest

extra coin to plate their weapons with silver. You can silver a single weapon or ten pieces of ammunition for 100 gp. This cost represents not only the price of the silver, but the time and expertise needed to add silver to the weapon without making it less effective.

2. Adamantine. From Xanathar's Guide to Everything (page 78):

   Adamantine is an ultrahard metal found in meteorites and extraordinary

mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons. Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

Any other options would be completely up to the individual DM's discretion.

As to why there are so few RAW options to improve weapons, I believe that the answer is bounded accuracy.

For RAW, the only way to explicitly improve an existing weapon is to coat it with either:

1. Silver - see PJRZ's or Darklordofsword's answer above

2. Adamantine. From Xanathar's Guide to Everything (page 78):

   Adamantine is an ultrahard metal found in meteorites and extraordinary

mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons. Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

See this answer

Any other options would be completely up to the individual DM's discretion.

For RAW, the only way to explicitly improve an existing weapon is to coat it with either:

1. Silver

   Some monsters that have immunity or resistance to nonmagical weapons are susceptible to silver weapons, so cautious adventurers invest

extra coin to plate their weapons with silver. You can silver a single weapon or ten pieces of ammunition for 100 gp. This cost represents not only the price of the silver, but the time and expertise needed to add silver to the weapon without making it less effective.

2. Adamantine. From Xanathar's Guide to Everything (page 78):

   Adamantine is an ultrahard metal found in meteorites and extraordinary

mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons. Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

Any other options would be completely up to the individual DM's discretion.