Titanium shouldn't be a fuel, more like a catalyst for the magic. So for powerful spells as fireballs it would require the caster to hold a big object, like a wand (mage), but for small boosts it would work well with a ring or amulet (bard, warrior). Also titanium-enhanced weapons should be useful.
Now combining the elements is a great idea. Name the kinds of Titanium with colours to not bind them with one single element.
- Red Titanium - Vitality
- Yellow Titanium - Agility
- Blue Titanium - Spirit
- Green Titanium - Stamina
Fire = R + R
Thunder = R + Y
Wind = Y + Y
Ice = Y + B
Water = B + B
Nature = B + G
Earth = G + G
Death = G + R
Every artifact (weapon, ring, shield) has a number of slots. For example:
An Old Magical Sword has one slot.
Adding R titanium increases its attack power.
Adding Y titanium adds the statistics (DEX ang AGI) bonus.
Adding B titanium adds the MP and INT bonus.
Adding G titanium adds the HP/DEF bonus.
A Fine Magical Shield has two slots.
Adding B+Y adds the STR bonus and "Frost Ward" spell.
Adding G+G increases the shield's defence and adds "Stonewall" spell.
An Apprentice's Staff has three slots.
Adding R+R+R gives the player spells "Firebolt" (R+R) and "Fireball" (R+R+R)
Adding R+G+G gives the player spells "Curse" (G+R), "Stun" (G+G) and "Poison" (G+G+R)
Adding R+B+Y unlucks spells "Shock" (R+Y), "Freeze" (Y+B), "Stupefy" (R+B) and "Barrierbreak" (R+B+Y)
Also, the titanium boosts the stats.
Overall:
Adding titanium to a piece of equipment boost its stats and unlocks spells. Only one piece of titanium just increases the stats, but from two to four-five it unlocks useful spells.
Type of spell depends on the kind of equipment:
Weapon - attack, curse, and attack boost spells (offensive)
Offhand - shield, healing and other boost spells
Rings - various effects
Armor - physical-boost spells
Helmet - magic-boost spells
Of course "titanium slots" would be available only for metal objects, so a mage's robe can't unleash the "Stoneskin" spell.
"Magical" artifacts such as wands, spellbooks and rings should be more "titanium-able". So an Ultimate Archmage's Staff would have 6 slots and be able to hold RGRGRG combination being a call for a set of some lesser spells, "Fireball" (RRR), "Shockwave" (GGG), "Curse" (RG), "Death Coil" (RGRG) and "Breath of Death" (RGRGRG); or YYYYYY, unleashing all offensive pure-wind-based spells. IMHO the number of slots might be as well limited to 4 or 5, except you like overpowered omniscent elementalists.
What do you think about that?