For what it's worth, the thresholds are only for pets and not for other unique drops. It wasn't completely clear if that's what you meant or not, so I wanted to address it.
As to the second point about a standard ratio - are you saying that the kill count needed for each boss to trigger a threshold should be the same for all? So for example, 500 kills would hit the threshold for both Nex and and 500 kills would do it for Dagannoth Kings? Or are you saying that a kill threshold would lower the chance of a unique item the same between one boss to the next? For example, 500 kills for Nex would increase the chance of a Torva plate by 15%, just as 500 DK kills would increase the chance of a ring drop by 15%? Either way, there's a problem with having an across the board threshold for everything since getting X kills for one boss isn't going to be the same level of difficulty or time needed as others. Per my examples, you could have several thousand kills at Dagannoth Kings by the time you could get 500 kills at Nex.
A few more questions to consider
Would the threshold be applied to all drops for that particular boss? or would each item have its own unique drop rate change?
Would it get reset after a drop is obtained? If not, wouldn't there be a problem where people who have much higher chances, and therefore items more quickly, cause those with lower thresholds but get a good drop to be denied a decent price for it? Basically, would there not end up being an inflation problem?
Would 'Threshold leeches' be permitted? That is, where someone with a high threshold comes along with a party and gets the kills but allows the others to pick up the items?