So what if we tether it to XP only in the same way that "minions" and "solos" are tethered to XP?
That is, say a "standard encounter" at level 1 is 100 XP.
A beholder is worth 100 XP, and designed to be encountered alone.
A goblin is worth 25 XP, and designed to be encoutnered in a group.
Fight one goblin, it's 25 XP.
Fight four, it's 100 XP.
So, your bugbear might be worth 100 XP. But by level four, the "standard encounter" has balooned to, say, 400 XP.
This is really almost exactly the same as 4e's current minion/solo rules, it's just expanded so that an "encounter budget" is only one part of a bigger whole -- the "adventure budget."
Before I comment on it I just want to check I've got your concept right.
So what your presenting here is something like you have an adventure budget of, say, 1000xp, That can then be broken down into various encounters of different xp values? Does this include things like traps and other encounter-ish things like that?
Have I got your idea right?