Hussar said:
Once you've laid all that out, then you can show what the baseline assumptions give you. And, because it's so out in the open, it should be fairly obvious what happens if you vary those assumptions.
See, as far as I can tell, 3e tried this, and the result was the rather justly-criticized CR system. It hard-coded things like the
fly spell, magic items, and whatnot, right into the baseline assumption. And those aren't exceptional assumptions to make: most parties probably have magic items, and probably fit the four main roles in a party. But then you get expected wealth-by-level guidelines and CR changes if you're off. And if you change the amount of magic in the world, the CR changes. And if you just lack a wizard, or a rogue, the CR changes.
4e tried to address that with roles, lower overall power, and a more granular XP budget, so that the assumptions were more easily met, but 4e also has pretty stringent requirements for "balance": 5 monsters. 5 characters. Solos with action-recovery powers. Open terrain. Etc. And some of the trade-offs that 4e made (roles, low-powered noncombat magic) aren't very well accepted.
I like the idea of leaving it a little more free and open, and maybe [MENTION=6684526]GreyICE[/MENTION] is onto something with the idea of just making it broad guidelines. If you combine that with [MENTION=28243]Joseph[/MENTION]Bear 's idea of making it the players' job to figure out how to overcome a challenge, you might have something that's robust enough for a lot of games.
Still, there's a price though. You can't account for which encounters will be climactic and which will be simple very easily. Your ability to plan out a 4e-style mega-battle is constrained, since your only tools are relatively blunt instruments until you just learn what your party can take.
Maybe that's fine, though? I dunno. D&D has always been a weird game with regards to "challenge" and "victory." Maybe weakening the game-style requirement of having a precise mathematical challenge will be fine. I'm not sure it'll make those who love 4e's precision very happy, though...
And, unlike with many places in the rules, I'm not sure this can be easily adjusted to be different for different groups. But perhaps there's a way I'm not seeing...hmm..