For personalized skills, you first need to consider two things:
1. What is the character using the skill like. What are their likes, dislikes, personality? Are they a flashy person who likes to show off, or are they a hardened person who prefers decisiveness and speed over flair?
2. How do you want your Classes to play? Is your Mage a Glass Cannon? Is your Knight imbued with elemental attacks?
When you answer these two questions, you can essentially answer "what skills should I make?". I know, that sounds simplistic, but it tends to be true. Just figure out who your characters are and what classes they will be. Then, take into account their personalities and how they might apply that personality to their Class (or what is essentially their actual job that they would get paid for if your fantasy world were real). Once you do that, it should be fairly easy to figure out which Classes and Characters take on which roles in battle. Maybe some of your elements are even split up amongst multiple classes as some of the disciplines are harder to master (or impossible) for people with certain personalities, because it conflicts with the principles of the element (Think Aang from Avatar Last Airbender and his trouble with mastering Earthbending because he was an Airbender and his teachings involved dodging, approaching a situation from a different direction, employing tricks, and using your opponents' moves against them. He couldn't get the hang of "direct attacks" that Earth employed because his attacks were all indirect or counters.).
You also need to decide a few things about the setting as well as the gameplay. How powerful with status effects be? How common is magic? How common is healing magic? How much healing can be done with magic? How hard is it to harness skills and abilities? What are the costs involved? Setting plays a large role in determining what skills you create as well.
I'll give you some examples from my own game to try to get you started.
Soldier
Magic Knight
Witch
Paladin
Thief
Pirate
Cleric
Ranger
Necromancer
Seems like a lot of classes, doesn't it? It's actually just the right amount for how my game plays. It also ties into the storyline quite well. But, that's neither here nor there.
Soldier has the skillset of "Adrenaline". Most of his skills revolve around buffing himself, healing himself, and dealing out massive chunks of damage. These skills all take a measure of TP, which must be built up over the course of a battle, unless you find good equipment that preserves TP. Some of his more powerful skills also drop his HP by % amounts, which can kill him if used improperly. The only "Multi-target" ability he has is simply a Battle Cry, which buffs ATK of his teammates.
Magic Knight has a skillset of both physical and magical blows. Some of these skills are dependent on which weapons he is currently holding in hand, or which elemental abilities he has mastered. He is capable of learning a physical elemental attack for every single element save for "Life" or "Death". He gets to choose which of those two he wants later. Most of his skills are single target, though there are a couple that are multiple target.
Witch has all our standard spells. All she can do is cast spells. Though, she can be tailored to one of two different ways of playing. She can be turned into a "Utility Mage" who delights in adding status effects to enemies as well as making those effects really hurt. Or, if a player prefers, she can be tailored to cast fairly powerful and strong magic that can wipe out enemies in single hits. She only has access to elemental magic. Though, she can't learn "Nature" magic.
Paladin has a skillset that relies entirely upon buffing the team. The other skills offer resistances to elements or types of damage as well as abilities to demolish undead or dead beings. The Paladin is not a tank, though can be played as one. The Paladin also does not heal his party. He merely prevents damage done to the team.
Thief has a skillset largely revolving around doing evil little backstabby thief things. Inflicting status ailments is most common for this class. Though, there are skills that utilize his Agility stat to do some fairly massive damage. He's typically used for quick precision strikes or single-unit debilitations that poison or stun or sometimes cause bleeds.
Pirate is my resident "nuke" character. That is to say, he carries a lot of abilities that revolve around doing lots of damage to multiple enemies. He doesn't have a single "single target" skill in his list. His job is crowd control and quick damage. He does have weaknesses (namely against magic or agility type enemies), but in some situations, he's who you take to do a lot of damage in a short amount of time.
Cleric has no healing skills what-so-ever. So, what does my Cleric do? Cleric has skills that remove buffs from the enemy as well as provide cures to debuffs on the party. The Cleric is also immune to ALL status effects. Cleric can single-target enemies and multi-target allies. MP Cost for curing allies is quite high though.
Ranger does a few things fairly well. She is all about being a "Jack of all trades". She's got a few debuffs, a few buffs, can single-target, multi-target, and sometimes heal the party for paltry amounts of HP. Oh, and she's the only one who can initially cast Nature Magic. She has skills for swords, daggers, long bows, and short bows. Her skills are dependent on what you equip her with, which means she'll play according to how the player would want to play her.
Necromancer is the only one in the game with a mastery of the "Death" skills. These skills typically are vampiric in nature and steal either HP or MP. Some of them can also result in instant death for enemies who aren't immune. These spells are also some of the most damaging in the game. The downside being that they are also some of the most costly in the game to cast with very few spells even coming close to the costs. Most of these spells are single-target and stay that way.
So, you see, it's just a matter of deciding on a few things before deciding what skills to make. Skills depend on how you have your setting set up, on how you have the personalities of your characters set up, and how those characters execute their jobs. It also boils down to what you'd like to do with the classes and what role you'd actually like them to fill in order to make skills for them.
You can easily start by setting up some "ground rules" for the class. What can they cast, what can't they cast? What types of skills do they learn? What can't they learn? Why? Why not? Set up some arbitrary parameters having to do with your classes. Just like I have one with my Soldier Class in that it can only heal or buff itself and nobody else. Just establish some ground rules for yourself and then try to work with or around those rules and encorporate that into the storyline.
