just brainstorming by writing any posibility i can think of without thinking about if it makes sense for us or not

using

Secret of Mana approach

hold attack button to charge. the longer you charge the better the attack when you release it.

SD3 approach

successful normal attacks charge a combo bar which can be emptied to use a special attack

usual MMORPG approach

click icon or press hotkey to use special attack. each attack has a cooldown (or not). each use uses up mana (or not).

Street Fighter approach

special attacks are activated by more or less complicated finger acrobatics. they can be used without restrictions but you lose control over the character for the duration of the attack.

Mega Man approach

every special attack has a limited number of uses which can be recharged.

obtaining

learning by leveling

reach skill level X and you learn a special attack

skill tree

every skill has an ability tree. you get a skill-specific ability point on each levelup you can spend on a special attack of your choice (passive boni, too)

learning by quest

Do a quest and get teached a new special attack as a reward.

design your own special attacks

players can design their own special attacks within certain limitations

blue mage approach

learn a special attack by getting hit by it (even in PvP???)

item-based

your equipment defines what special attacks can be performed