Magic items to modify existing items?

[Oramac]

How would you do this?

For example: if I give the Ranger a long bow that does NOT offer any kind of attack/damage bonus, but does create its own arrows when the bowstring is pulled, then later wanted to give the option of making it a +1 bow, how would you do this? Magical Bow Oil? Spells?

This is just an example. Any other "upgrade" ideas are welcome!

[Maxilian]

There are some options:

-He could find a Rune (Like the ones that are for the UA Rune Master -They were redisigned as magic items, but haven't seen them)

-He could use a Magic Forge (You know... the whole plot of the Lost Mines of Phandelver :P)

-He could get it Enchanted by a Wizard.... for a price

-He/she could modify the bow with the help of a Gnome inventor

-They could get a blessing (So the power comes more from them and not the bow itself)

etc.

[Coffee_Dragon]

Psicrystal that embeds itself when properly applied.

Blessing from god or other powerful being (who is apparently a big jerk for not just blessing the character directly).

Items picks up magical aura from being subjected to sustained unnatural influence.

Character harmonizes with item to discover/awaken new properties.

Item levels up.

[CursedRhubarb]

After defeating a strong magical creature or being subjected to a strong magical force (special mob or after they trigger something like a glyph of warding and get hit with a spell) some of the magical energy appears to have been absorbed by the bow, increasing its power and granting a +1 bonus. Could also use it to grant an effect like the old 3.5 effects that add 1d6 elemental damage or other effects.

[Coffee_Dragon]

Character inadvertently imbues weapon with own life force. Item gains +1 permanently, but character gets 1 level of exhaustion... also permanently!!! No, just kidding, it goes away later probably. Character starts thinking of monetizing this process.

[Bohandas]

Does 5e still have weapon and armor crystals?