Hello, and welcome to FFH, Agentlovac! It's always nice to see people interested in modding FFTA/FFTA2. First off, please don't double post. You can edit your posts using the button at the upper right hand corner of your post.
Secondly, attached to this post is a folder containing all of the tools I use for modding FFTA2. This includes (importantly), the FFTA2 Spreadsheets, which are a goldmine of information and addresses, the hex editor Goldfinger, the text table to use with it, Blanky's equipment editing module with Nightmare 2, and the Job/Ability editors. They should be fairly obvious in how to use them, but if you have any questions, let us know!
Now, to address your individual points...
-Things like negative stats, auto statuses, and accuracy modifications on equipment is not possible.
-Units in FFTA2 GG start with max MP and MP does not regenerate.
-Fire/Thunder/Blizzard magicks have their own individual gimmicks in most mods I make, including A2 GG.
-Geomancers in A2 have access to Fire, Water, Ice, and Dark. This means that with the Vartan's Wind, you've got quite a few elements covered.
-Giving HP/MP bonuses to gear is not currently possible.
-You can easily create new Dualcasting abilities. Dualcast still exists in A2 GG, as does Storm. Storm is just a version of Dualcast, but I limited it so Gale, Cyclone, and Whirlwind are the only Dualcastable skills that the Gria race has access to.
Today's new job is the Samurai, taken from FFT/FFTA GG! Not fancy by any means, but the Samurai acts as a hybrid job for Humes, usable in different ways depending on how you've grown your unit. They can smack things in the face with their katanas for heavy damage if you've used physical jobs, or you can use Iaido skills to magickally destroy nearby enemies if you've been raising your Hume as a mage. Samurai are evasive tanks, being able to be thrust into the thick of battle and easily evade attacks. One of their Iaido skills, Adazakura, even boosts their Evasion and Critical Hit rate.
Murasame: Restores the HP of all units surrounding the user in a large area, Water Elemental.
Osafune: Damages the MP of all units surrounding the user.
Ashura: Deals Fire damage in a line in front of the user.
Nosada: Bestows Protect and/or Shell on all units surrounding the user in a large area, may flat-out miss a unit.
Kotetsu: Deals Dark damage and may Blind units surrounding the user.
Ama-no-Murakumo: Deals Wind damage and may Slow units surrounding the user.
Adazakura: Bestows Evasion Up and Critical Up on the user.
Sumihomura: Deals tremendous damage to units surrounding the user in a large area, may hit the user, but can be evaded.http://imgur.com/a/5fBkD
FFTA2 Modding Tools: https://www.sendspace.com/file/ua53zg