Some percentage of their revenue proportional to the use of the copyrighted material.

I don't mean that as a pat answer: that's the actual formula I'm proposing. Usually laws are written in dollar amounts for infringement which then affect the licensing fee charged. I think that assessing infringement or licensing on a percentage basis is much fairer, and that way artists who sample can do so without worrying about big licensing or legal fees, because they won't be liable until such time as they start seeing profits, at which point some fraction of those are shared with the other 'contributors.' This isn't a perfect system (I can think of many reasons against compulsory licensing, and the idea of price controls for copyright holders is also problematic), but at least it means that there's some linkage between clearance fees and actual economic benefit.

Like I said, a small mechanical royalty for obscure songs where you don't know who owns what, or how to pay them. No copyright trolling.

And for big songs, call it gold-record hits and above, whatever Jay-Z (or equivalent) demands. If you want to sample a big band's big hit, you are arguably getting the better of the arrangement, and they ought to have the right to charge you whatever the market will bear. Or tell you to shove it if they don't like the song they're getting shoved into.