Alright, i've got some suggestions
First, We have the sinful trait, I don't know what it does, seemingly it gives you a lot of Ki and a ton of health currently. also, i don't know how it's coded in, but maybe have it be a permanent buff, rather then reapplying the buff constantly. (In multiplayer if it's constantly reapplying the buff it likely eventually causes a crash from information overload, i only assume this because consistently my friends crash after i got the sinful debuff). anyways I suggest that sinful gets changed eventually base on whatever sin you unlock, and gives bonuses to reflect that, on top of a universal bonus that applies to all the sins. (I don't know the scope of your coding ability, or limitations of the game, so bare with me)

Second, Rather than having one form, i would suggest two or three, the first form is tied to your sin and gets stronger gradually by killing bosses, and it's a free cost form that you can stack the others on top of, the second form, is stronger medium cost form that makes you overall stronger, and third is a "Demon Lord" or "Devil" form that his very strong with a high cost.
(Also maybe make the background of the aura Translucent or Transparent, so it doesn't block things.)

Third, the actual sins themselves, so to set this apart from the other form mods, i think we can tie these ones to other alternative classes as well as ki so your not locked into one playstyle, and these buffs can be applied either in the form of the persistent first form or the trait buff, though i would reserve the trait buff for the universal buff that the trait provides.

Sloth (Dark Blue Water Aura)
doing nothing for extender periods of time, Sleeping a lot, having summoned enemies doing damage
It represents laziness or the refusal to work or do things yourself.
possibly have it boost Summon damage (?) or convert ki damage blasts into summon damage. Damage Boost

Greed (Shiny Gold Aura)
Looting chest constantly, hoarding money, and selling items
Pretty clear cut, the desire to always have more.
Increases drop rates of money, maybe tied to Ranged damage (?), because ammo is expensive.
Damage Boost

Envy (Green aura)
Envy can be acquired by looting bosses, buying goods from vendors, or taking furniture from structures like the dungeon
Representing that your jealous of others for one reason or another to often a destructive fault.
Increases rare item drop chances, tied to Range Damage (?)
Damage Boost

Gluttony (Orange Aura)
Gluttony points can be acquired by Eating, Drinking potions, spending long periods of time in a single biome.
Gluttony is the idea of enjoying things, often food or potion in excess, basically too much of a good thing can be a bad thing.
Increases the effects of consumable items, and maybe biome specific perks. tied to Summon Damage (?)
Damage Boost

Wrath (Flaming Red Aura)
Wrath Points can be gained by dying and killing large amounts of one enemy (like the banners)
Wrath represents uncontrollable anger and aggression, this is represented very well in the form killing a lot of things or getting annoyed that you keep dying to a mob or a boss. Damage boost (stroger boost against bosses) tied to Melee Damage (?)

Lust (Flaming Pink Aura)
Lust points can be accumulated by receiving healing from the nurse (old game reference), and interacting with NPCs frequently.
Lust represents often uncontrollable desire and unrelenting pursuit of pleasure of any kind.
Defense Boost, (if your enemies love you, they would take it easy on you), possibly a spike charm affect, and damage boost, tied to Magic or Melee (?)

Pride (Watery Purple Aura)
Pride Points can be accumulated by using magic, collecting Stars, and crafting weapons.
Pride is extreme self love in the form of superiority, and the refusal of aid from others, often divine will, this is reflected by using magic and taking pride in your own work, as magic is pure power and miracles that you perform on your own.
Better chances at higher prefixes like legendary, maybe even applying legendary to other things besides melee weapons, (possibly even better crafting as a whole), and damage boost. Tied to Magic Damage.

Conclusion
Overall these are the suggestions I think would be cool to both give each sin a unique playstyle, and to set the overall trait mod apart from the standard trait mods, of course, you can also have it boost ki damage as well to keep that available, or you can convert Ki damage into another damage type, given the already existing buffs to Ki, it might be better to have the Sins dual boost Ki, and their respective damage type, anyways, i appreciate you responding to me, even about a year later, and I hope this gives you some ideas. I wish you the best of luck and hope to try out the new version of the mod~!