Could their be "% target's lvl 1 max base health" as a damage source?
Almost like an inverse of % max health, where the amount of base health your target has at lvl 1 (base health mind you, so before items, masteries, and runes are factored in) is the "bonus" damage source for your ability. It would make the lvl 1-3 trades pretty strong, but as the game progressed, the strength of that damage would "weaken" (in comparison since the actual amount of damage would remain consistent throughout the entire game) as your targets gained more health through items, leveling, and whatnot. This could make more tankier champions (Poppy, Shen, Leona, Maokai, etc...) have a strong early game damage source, but unable to carry that early game power into late game.
For example, Poppy, who deals 7% targets max health (TMH) on her Q, on top of some base damage and +80% bonus AD, could work with this. Dealing 7% TMH every 5s is pretty powerful, even more so since likely she will have some CDR from 
and maybe also from 
or 
, making it closer to or exactly a 3s cooldown. So instead of having it be the 7% TMH, it now is say 10% or 15% target's max level 1 base health. (I increased the percentage just to add some extra oomph in the early game since she will not be dealing very much later on with this kind of damage source).
% TMH is a good way to scale a damage source throughout the entire game to be a consistent damage source without forcing any particular item builds. However, the one issue with this is that while the targets health will increase, other resistances (AR and MR) don't necessarily scale the same. So while you always will deal 10% TMH (as either bonus physical or bonus magical damage, i'm not counting true damage with this), how hard that hits is more dependent on how much AR or MR the target has respectively. If you have 1000 Health, 10% TMH will deal 100 damage to you, but that 100 damage will be effected by resistance. So if the damage is calculated as physical damage, someone with 30 AR will be damaged more than someone with the same amount of health but has 50 AR (MR if the damage is calculated as magical).
