So hear me out. Each of the jungle items has a % based form of scaling except for warriors, why is this important?
% based stats tend to be more relevant than flat stats in the game because they scale into late game. each of the current jungle enchantments has late game scaling because of this.
**Cinderhulk** has +15% bonus health causing any health you buy after it to be worth more as you get more HP.
**Runeflaive** has a SPELLBLADE effect that has 100% base AD scaling and 30% AP scaling, this gain's strength each time you level up and each time you buy AP so again it scales into the late game.
**Devourer** gains stack's passively as the game progresses, this hits a cap but the effect is still the same, an item that gains stacks into the late game.
**Warrior's** is the only enchantment that has pure flat value. riot experimented with adding a true damage burn similar to the old spirit of the elder lizard item but due to it being open to abuse by layer's it wasn't included.
I see no reason why a 5-10% armor penetration stat cannot be added to the item in place of its current flat +10 armor pen. it would effectly do the same thing as each other enchantment, give a stat that increases in gold effectiness as the game continues.
The AD on the item can of course be adjusted to compensate for any bonus power this may give early.
If it seems the item may be abused by laner's another solution would be to have the item start out at 1% armor pen and add +0.5%/+1% armor pen per large monster killed up to a cap at 10%, laner's would have to waste time farming nearby jungle camps to gain the bonus deterring them from even bothering to buy the item.