For example, Throwing Dagger/Axe weapons that can be equipped in stacks, they'd require both throwing and sword/axe to use effeiciently. 

Different shield types that might amplify certain damage types, like a bashing shield that requires mace and shield skill.

Could also put stat requirements on certain new weapons/armors.

With new armor/weapons coming out, they could fit a good/evil theme and require the associated skills to use efficiently, like a Doom Axe or Holy Sword or Vampire Flail, that require Necromancy/Prayer/Lifetap, staves and robes that require mundane/white/black magics, magical bows that need archery and druidry etc.

On top of that ,maybe new random enchants on gear that tie to certain skills. Like a monster drops Enchanted Cleansing Plate Armor, it has a chance to auto-cure poison every tick, but requires Poison Resist to use. Or a monster drops a Crossbow of Hex, has a chance to cast Curse on your target when you shoot them and it requires archery and necromancy. Some of these skills are much harder to level up compared to normal weapon skills.

More examples of random enchants that could be found on items.

Rage, weapon has double attack chance, requires adrenalin

Parry, weapon can block attacks, requires Protection

Slow, weapon can cast Slow on target, requires White Magic

Energize, armor with chance to give increased mana per tick, requires certain int/medition or mundane magic.

Just interesting stuff that adds a 4th element to gear you find (material quality, enchantment level, magic effect, then skill tie in)