Righto I'll start off with character creation as my first suggestion/revamp.
Giants - They're fine as I see them. Though, with the 0-51 coding in. Giants can be attacked right away by people who attack new characters, because they start off at level 90.
Fishmen - They should have an option for what they want to look like.
Aarlong - They should have a special ability to pull out their teeth to cause Health damage with each Punch. Skill name: Bite Punch
Hachi - they should have a lower delay in their attack speed with every weapon, half the usual attack speed for everything.
The third icon that I can't remember the name it's based off - They should receive a larger speed boost then all the other types in the water(About 1.5x larger)
Each type of fishman will receive a speed boost of double their speed.(The Finned one gets a 3x speed boost.)
Humans - Righto, this is going to be my favourite to type out, I guess.
Humans get a chance at being born with an 'Atypical Body Structure'. Making their bodies a little different to most, Mainly when it comes to Devil Fruit. Being born with the ABS will make your character have 20 skill points at the start, a small boost for being different. They also have the ability to use an unknown method of stealing a Devil Fruit Power from someone else, effectively destroying that person's DF, or they can eat a second one. Each method is different, they have a high chance of failure which results in your character losing alot of lives(it automatically sets you back to 1 life, for a real risk.).
Stealing a DF Power removes it from the person you steal it from, if you succeed. However there's a 50% chance of failure. Because it's 50-50 the Devil Fruit's power won't be as strong as you eating it for your first, you'll miss out on one of the skills if it has multiple techniques, or the boosts it gives are far less.
Eating a second fruit, has a 75% chance of failure. Because of this, you get the raw untapped power of this fruit. Meaning you get the full boosts and everything.
Two Logia based fruits cannot be used at the same time. Meaning no Yami Yami + Flare Flare
Two Zoan based cannot be used at the same time either. Meaning no Phoenix Phoenix + Dino Dino
Two Paramecia however, can be used. e.g; Gomu Gomu + Diamond Diamond.
Zoan + Logia
Zoan + Paramecia
Paramecia + Paramecia
They are the only combos that can work at the same time.
Devil Fruit now.
I think you should remove some of the incompleted fruits from availability. E.g; Jackel Jackel doesn't have an Icon yet.
Some of the new fruit should be put in. Like Numa Numa no Mi(Swamp Swamp Fruit) which is a logia, basically give them a muddy-brownish-purple coloured liquid that forms over their icon.
Numa Numa no Mi allows you to make swamps which can pull in people(Not like Yami Yami.). No matter how small the swamp looks, it delays your movement, making it harder to escape.
Magellen's fruit (Can't remember the name) should do a willpower based DoT attack with it's 'Poison Form'. Each attack will cause usr.Willpower damage over time. Meaning someone with alot of willpower can take out someone who's really strong. The poison will last for about 30 seconds.
That's all I can think of for the Devil Fruit at this time.
Stats
The only quarrel I have with the stats? the inability to dodge naturally. I mean if you're incredibly fast, you should be able to dodge the attacks of some people who are slower.
Basically a Speed vs Speed test. Meaning speed boosting DF will have a second reason.
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And that is all I can think of for my suggestions.