Some time ago I had an idea of an action game in wich the plater controls a wizard. To cast the spells the character pronunces a series of magic words. Each magic word is a pronounced pressing a button, and after the combo of buttons is succesfully inputed the spell is cast. Now, I don't remember seen this before, I mean, you must perform the full combo to make one move (fireball, heal, magi shield), do you think this could work?
The first spells would be combos of 2 or 3 buttons, and the most advanced around 8, so there's a balance bethween the basic and the powerful.
Feedback apreciated.

Combo systems are not uncommon but consider the pacing of the game. If it is very action based then this system slows down the game play. If it is a slower plot driven game then this system may not even be deep enough.

Combos themselves though have been done in various ways before, and key pressing is not the best idea for one. For one thing it takes up a lot of keys, for another, its no more interesting or immersive to the player than a typing test. Interesting combo systems would involve things such as collecting regents for spells, setting up rituals, or having environmental aspects combo with spells to strengthen or weaken them.

I built a combat system like this before and have seen it before too. Xenosaga 1, 2 & 3 use a spell combo system. There's a DS RPG that uses one, but I don't remember it's name. Xenogears used an ability combo system and I think the MMO Warhammer also used one. I love these kind of battle systems.

I disagree with JasRonq and feel that key pressing makes the combo system more interesting. Rituals and environment buffs are good ideas too though.

It can work in fast paced action games, but you need to make it easy for the user to input the combo. Plus it can be hard to remember all the combos.

To solve this in my combat system, I had the UI show the user possible spell completions of spells that he had already learned as he entered the combo. Of course, I limited the list to only show up once there were only 4 or 5 spells to complete from a given combo to prevent a very messy looking UI.

Spellcast uses a similar system, though instead of words you use gestures. The neat gimmick here is that you can flow one spell into another. For example, the last gesture of an attack spell might involve a pointing index finger, which is also the first gesture of a healing spell, say. Spellcast is a turn-based combat game, so much of the gameplay comes from examining your opponents' gestures and trying to guess what spells they're planning on casting.

Check out Avencast' system it has a book of spells that all have number combos. Its very similar to the game you describing (plus it's a pretty good game too =P ). It actually allows the player to be able to cast a large collection of different spells without breaking game play. Granted the game as a whole isn't fantastic.. but the gameplay is really enjoyable.