I got my inspiration for a large variety of skills primarily from dungeons and dragons and from games like Heroes of Might and Magic and Age of Wonders.
In Age of Wonders you can select from a variety of different skills whenever you level up. In Heroes of Might and Magic you can have different degrees of a skill (like Basic Leadership, Advanced Leadership, and Expert Leadership). I incorporated both these ideas into my game.
So I came up with a couple hundred different skills. Stuff like Archery 1, 2, and 3 that increase your ranged attack ability, to things like Healing Touch 1 & 2, to Regeneration, to Death strike 1-3 (death damage inflicted with melee attacks, to damage attacks like Flame Blade & Tidal Wave, to Berserk abilities, to Spell Mastery (two spells per round), to artifacts that may be summoned, etc.
Again, getting ahead of myself. Besides these "skills" I also implemented "Weapon Skills" as a concept. This means that when you advance a level each character can add a given # of points to a variety of weapons.
This increases the # of attacks and damage with that weapon. It also can gain you additional attacks after enough points are spent. It also grants you a skill title to that weapon such as proficient, specialized, and master.
So this meant I already had "Skills" so I changed the name of the other skill to "Disciplines".
Here are some samples of disciplines, including some progression.
Disciplines can either be active or inactive. Active disciplines give the benefit once the character enters battle. Like Archery 1-3 for example. You gain the additional attack/damage once the character is summoned.
Inactive disciplines must be activated. This is done much like a spell. You click on the spell book image of the particular discipline and bring the power into the game. This can be something like a damage power (fire flash for example) or when summoning an artifact, or something beneficial like healing or an ability increase.
Only Immortals may have disciplines. Immortals are kind of like rpg character classes (like knights, warriors, priests, etc...which I will go into more later). Creatures cannot have disciplines, although they can and do have "Creature Abilities".
Disciplines are each worth a certain amount of points. The better the discipline the more it costs. Immortals start the game with 6 points and gain 4 per level. Selecting disciplines is often based on your character class and what kind of character you are looking to create. The # of disciplines allows for a vast array of different character types with unique powers and abilities.