Maybe T4 item that you can "assemble" from battle vest and enchanter's emblem?

[Oleg-fav]

Just like Leech, but instead of "omnivamp" it gives you stats depending on your current health - the healthier you are, the more stats you get. Something like that:

Prosperity
+30% bullet resist
+30% spirit resist
-20% ability cooldown
+20 spirit power
+50% weapon damage
+30% fire rate
Passive: For every 10% missed health becomes 15% less effective, losing 15% of all it's stats

 

[nathanieljbrill]

Hey, that's pretty cool! Way less cooldown though that's OP. Most of the time your ult is on cooldown you're at full HP. I'd change this item up into:

Conditional: 35%, 35%, 10%, 25, 30, 20, passive: the stat bonuses of this item are reduced by the percentage of your Maximum HP that you are missing.

 

[AleksiyFlek$$iy]

I like this idea. The item is kind of inversed Frenzy.

But I don't know how useful this item would be. These stats on max health are enormous, but it's unlikely that you will be able to keep HP on same level.

 

[gibnewitems]

Part of what keeps vest and emblem balanced is the fact that they don't build up into anything. Then again, 10% of any hero's health bar (even late game) is nothing, so any amount of chip damage or stray poke pretty much invalidates any stat boost from this item. You're better off buying CD item, even if you have both vest and emblem.