The Sigil
Mr. 3000 (Words per post)
Going back to the OP, I'll say that Pathfinder does have one of the most interesting systems for magic item customization in the "Relics" rules but it does require some work on the part of the DM, and I've found is most effective when combined with the PF2E rune system to allow the Relic to "level up" in power with the character at the same rate as magic items would. Relics - Rules - Archives of Nethys: Pathfinder 2nd Edition Database
For example, here is a relic I created to give out to one of my low-level groups that I intend to grow with them through the campaign:
"Retribution" - This is a club made of fused obsidian with a grip wrapped in manticore hide. A shadowy flame appears to blaze within the club's smooth facets. (Relic Gifts: Sonic - used homebrew stuff found at PF2e Scribe, Shadow is in the core rules - Relics - Equipment - Archives of Nethys: Pathfinder 2nd Edition Database) - I combined the relic with "expected runes" for character levels (instead of making runes replace gifts) since players are expected to have these runes at these levels anyway. The "Enlarge" gift was one I made up because I thought it was cool.
To me, this feels like an "interesting" weapon that can grow with the party as they discover more of its powers. But perhaps my idea of "interesting" and yours are different.
For example, here is a relic I created to give out to one of my low-level groups that I intend to grow with them through the campaign:
"Retribution" - This is a club made of fused obsidian with a grip wrapped in manticore hide. A shadowy flame appears to blaze within the club's smooth facets. (Relic Gifts: Sonic - used homebrew stuff found at PF2e Scribe, Shadow is in the core rules - Relics - Equipment - Archives of Nethys: Pathfinder 2nd Edition Database) - I combined the relic with "expected runes" for character levels (instead of making runes replace gifts) since players are expected to have these runes at these levels anyway. The "Enlarge" gift was one I made up because I thought it was cool.
- Potency Rune +1 (+1 on attack rolls)
- 1st level: Enlarge (command word, 1 action): Retribution changes in size from a regular club to a greatclub. Its damage dice change from 1d6 to 1d10 but it must be wielded in two hands. In this form, it retains the club's "throw to a range of 10 feet" property. It can be returned to its normal size by means of a second command word (1 action).
- 1st level: Minor Shadow Gift: Obscure (passive): Retribution absorbs light and shadow around the bearer, hiding the bearer in darkness. As long as you are in dim light or an area with shadows, you can attempt a Stealth check to Hide, even if you aren't concealed against a creature, such as a creature with Darkvision.
- 3rd level: Gains "striking" rune (deals two damage dice on hit). Half of the damage (rounded down) is considered to be sonic damage.
- 5th level: Minor Sonic Gift: Amplify Fear for 1 minute (envision, 2 actions): You use subtle yet potent audio effects to amplify the emotional state of a Frightened creature within 30 feet. The target's Frightened condition increases by 1. You can target an additional creature with this effect once the relic has reached level 8 and an additional creature for each 4 levels thereafter.
- 7th Level: Potency bonus increases to +2
- 9th level: Major Sonic Gift: Echo Step 1/day (envision, command word, 2 actions): Your relic emits a varying cadence wherever you move. For 1 minute whenever you use an action with the move trait you create a single afterimage of yourself made of sound. You can have a maximum of 3 afterimages at any given time. These afterimages function as images made by the spell mirror image. Once your relic is 16th level you create two images per action with the move trait instead.
- 11th level: Gains the "greater striking" property (deals three damage dice on hit)
- 13th level: Major Shadow Gift: Dancing Shadow 1/hour (command, interact, 2 actions): Your relic's shadow detaches from your relic and dances through the air to attack an enemy. When you activate the relic, designate a target. The relic's shadow flies up to 60 feet until it is adjacent to that foe and then makes a Strike against it, dealing 5d8 damage on a success (double on a critical success). The damage type is of any type normally dealt by the relic (impact or sonic). The shadow uses your attack bonus with the relic, and it uses and contributes to your multiple attack penalty. While the activation is in effect, you can use a single action, which has the attack and concentration traits, to mentally direct the shadow to make another Strike against the same target. The shadow lasts until that target is reduced to 0 Hit Points, that target moves more than 400 feet from you, or the target moves to an area where no shadow could be cast, such as an area of complete darkness, whichever comes first. The shadow doesn't take up space, grant flanking, or have any other attributes a creature would, and it automatically follows the chosen foe. The shadow can't make any attack other than its Strike, and feats or spells that affect weapons do not apply to it.
- 15th level: Potency bonus increases to +3
- 17th level: Grand Sonic Gift: Sonic Boom 1/day (envision, command, actions): You release a torrent of violent sound. Each creature in a 120-foot cone takes 10d12 sonic damage (basic Fortitude save). On a failed save, the target is deafened for 1 hour.
- 19th level: Gains the "major striking" property (deals four damage dice on hit)
To me, this feels like an "interesting" weapon that can grow with the party as they discover more of its powers. But perhaps my idea of "interesting" and yours are different.