An Actual Devourer Fix Everyone Will Like
Devourer is a problem, and the design is generally flawed. Long ramp-up means the payoff has to be huge. Right now it's too huge, and some champs are able to minimize the ramp-up far better than others, leading to imbalance.
Riot should remove the delay/spike model and smooth it out with a linear power increase that scales with kills and monsters but also with items. A 100% on-hit increase every other hit averages out to a 50% increase to on-hit damage. Since we are removing the delay and spike, our goal is to increase earlier power while lowering the ceiling a bit.
The solution is this: simply increase the effect of on-hit effects from items by 1% per stack with max stacks of 30. You start with a base +10% and cap at a maximum of +40% on-hit from items. Devourer starts at 30 on-hit damage, gains +1 damage per stack, and benefits from the +% on-hit bonus.
The benefits here are many. First (and most importantly), it nerfs the ridiculous Devourer burst and champ ability cheese, most notably the horrendous interactions with
for example. Not to mention the bizarre application to 
Secondly, it smooths out the power curve while still having snowball potential. Right now junglers start off relatively weak and are incentivized to afk farm to hit the "unstoppable juggernaut" threshold as early as possible. With my idea, there's linear increase with farm with interval power spikes upon completion of each additional on-hit item. Most importantly, the base power of Devourer is slightly increased, and you're never too far behind the curve to have a meaningful lane presence.
Lastly, it opens up the option for Devourer to see use on new champs without drastically favoring champs with on-hit effects baked into their kits. By limiting the Devourer bonus to items only, it can apply to a wider range of items that includes
items. As a result, Devourer would appeal to ability-based burst champs who build
or
while giving a better-scaling end-game alternative to
users at the cost of gold and delayed power. True AA aspd on-hit champs that build
still benefit from stacking on-hit items. It also opens the door to a pure AP
build as a result of multiplicative % bonuses.
In conclusion, this is the kind of design we want to see from Riot that adds meaningful diversity to champ and item selection without encouraging bizarre play, breaking champ kits, and creating extreme outliers in payoff relative to the intended time investment.
TL;DR: Have Devourer scale linearly with items only to diversify champ usability and nerf the fuck out of broken Devourer/kit interactions.