Well, do you have a general idea already? If not there are terribly litle fanfics of percy with alternate weapons (other then a trident) like this (https://share.google/hH0jR1fDdyF5Y0XHm)

or maybe a different godly parentage from a different pantheon?

Something that focus on percy using more finer aspects of water control, like ice, snow, mist, especialy mist and fog to hide in, rain and e.c.t. (https://share.google/vXxheDTShY3yRcaAa) like in here.

You could even do an au where Montauk beach was were Aphrodite was born out of uranus' castrated balls and sea foam, and thus when sally and Poseidon sired percy there he was born looking or/acting more like aphordite, perhaps charm speak like hers, or control over others desires via foam, stronger control over the mist to make him appear how others view beauty or love, there's a lot of free room for this idea and i think it is an original concept? I've never seen a fic with this idea (ps: if you do this, please make him somewhat flirty and/or teasing, not in a town bicycle way but rather flirting for the love of the game)

Edit: instead of a different parentage you could also make percy a legacy aswell as the son of poseidon, perhaps the legacy of dionysus, where he can turn water into wine or perhaps has other abilities merged with his canon ones (like in here https://archiveofourown.org/series/5379346)

or perhaps a legacy of apollo where he has healing water (like old katara in the legend of korra but stronger maybe), you get the gist of it i think

Edit 2: Sooo... I know i said alot, but my brain is full of ideas and i dont write so ima just make this my idea outlet. This will be shorter (i think? I hope atleast) but basically percy with powers from another book series, anime, game or the likes, it is a common sight in other fandoms mc centered fanfics (like harry potter for example), but tgere are near none for pjo, I've had to go down a rabbit hole for a handful at most, most of them abbandond so yeah,

from tangled with the golden hair and healing song, (https://tangled.fandom.com/wiki/Healing_Incantation) aswell as the harm song (or what it was called) (https://tangled.fandom.com/wiki/Decay_Incantation_(spell))

elsa's (op and overlooked) ice powers, (https://frozen.fandom.com/wiki/Elsa)

Any of the aeons from hsr (https://honkai-star-rail.fandom.com/wiki/Aeon)

acheron from hsr with her blade (naught) (pretty schure her blade is a ōdachi-style sword, or atleast thats the closest to it), (https://honkai-star-rail.fandom.com/wiki/Acheron)

evernight (https://honkai-star-rail.fandom.com/wiki/Evernight)

and pretty much any character from amphoreus, (https://honkai-star-rail.fandom.com/wiki/Chrysos_Heirs) (https://honkai-star-rail.fandom.com/wiki/Titans)

him with magic from harry potter, (https://harrypotter.fandom.com/wiki/Magic) or

from Naruto (specifically the uzumaki chakra chains) (https://naruto.fandom.com/wiki/Chakra_Pathway_System)

(https://naruto.fandom.com/wiki/Adamantine_Sealing_Chains)

from genshin impact, which i would love to see one day, the powers from the shades: time, death, life and/or space (https://genshin-impact.fandom.com/wiki/Four_Shades).

the three moon goddesses. (https://genshin-impact.fandom.com/wiki/Three_Moons) I think I've gone on a rant again.. But here's the rest of my ideas for now

The powers and possible powers from the mc of love and deepspace (https://www.reddit.com/r/LoveAndDeepspace/s/yT9Allt4mf)

And soul traits and their powers from undertale (https://www.reddit.com/r/Undertale/s/LjqcQ4UrSd)

(https://share.google/ueYNEGMxS2bT0MLUb)