Item(s) that improve champion states by percentage similar to Rabadon Deathcap and Void Staff

The follwing are a list of (most) items that provide stats in the form of a percentage.

Attack Damage (AD)

• Sterak's Gage: +25% base AD. Extra +25% base AD after taking 400-1800 (based on level) damage in 5 seconds (unique passive, 45 second cooldown).

Ability Power (AP)

• Rabadon's Deathcap: +35% AP (unique passive).
  (Void Staff actually only gives +80 flat AP, you seem to have mistaken the 35% magic penetration it gives for something else.)

Cooldown Reduction

• Essence Reaver: Gain increasingly more cooldown reduction from critical strike chance provided by other sources (unique passive).
  This is not really providing cooldown reduction as a percentage of cooldown reduction, but I left it in here anyway.

Health

• Cinderhulk: +15% health

Other stats are always provided for by items as a percentage of base amount:

• Health Regen
• Mana Regen
• Movement speed (except from boots)
• Attack speed

If anyone else has any to add that I have forgotten, please say so.

All CDR (cool down reduction), items improve by a percentage, you can use your spells faster so it is a direct percentage change in average DPS (damage per second). This is the best bet for a percentage change in power on Ziggs after Rabadon's and Void. Remember though that you need a good base amount as well as a good percentage, percentage based stats are very powerful in league but they don't work if they don't have a solid base to multiply from.

Indeed, I can't comment on other's answers yet but in addition to being vague about CDR (Essence Reaver isn't really a good choice for Ziggs and other CDR items are still percentage based); you forgot critical damage.