I did like socketing in Diablo, especially the part where you would combine gems using the horadric cube, maybe they'll let us combine crystals here in ffxiv for various effects like:

Fire Crystal + Fire Crystal + Fire Crystal = Fire Cluster (Socket effect: Increased fire/melee/ranged damage OR Added fire effect)
Fire Cluster + Fire Cluster = Flame Cluster (Increased effect fire cluster)

3 Umbral type Crystals = Umbral Cluster (Added umbral effects)
etc.

Obviously different elements have different effects on melee/range (such as increasing defense(Earth), evasion(Wind), etc, but with spellcasters, it would simply add bonus effects to relevant spells.

Although obviously would require more than just 3 crystals cause that'd be too easy.