I like the idea of hidden bonuses. (though it will be unhidden in a matter of weeks)
For lack of a better term..."Synergy Bonus"

E.g. Put STR and DEX on a glove for an additional small bonus of VIT

Combination of materia sounds good too. Turn 2 low level materia into a higher level one at a chance of failure. That will use up a lot of low level materia.