On the 4E front, I spent a while last night banging around with test characters (thank you, Essentials, for making it sooooo much easier to create test characters and calculate their damage output) and a big ol' spreadsheet, and worked out what I think is a tolerable system to estimate "magic item power" as distinct from "character power." Here are a few of the things I came up with:
- Weapon that deals 1d6 bonus damage on a hit, once per round (2d6 at epic): +0.5 level
- Arcane implement that adds 2 bonus damage to all damage rolls (3 at paragon, 4 at epic): +0.5 level
- Increase the user's max hit points by 8 (14 at paragon, 20 at epic): +0.5 level
- Grant resist 4 all damage (6 at paragon, 8 at epic): +1 level
- 1/round when you hit a bloodied foe, deal 1d8 bonus damage and heal yourself the same amount (2d6 at paragon, 3d6 at epic): +1 level
So, for instance, if you're an 8th-level fighter wielding a
flame tongue sword which deals 1d6 bonus fire damage, and wearing a
periapt of endurance which gives you 8 extra hit points, you're effectively a 9th-level fighter for purposes of encounter difficulty. Obviously, I haven't had a chance to playtest any of this, but it looks in the right ballpark, at least*.
The approach I used was to create a set of "baseline characters"--a knight, a thief, a mage, and a warpriest, which are the Essentials versions of fighter, rogue, wizard, and cleric respectively. I then worked out estimates of how their hit points, defenses, damage output**, and attack bonuses would scale over the course of 30 levels. That enabled me to compare "level X character with item" to "level X+1 character without" and see how they stacked up... or, rather, to make the spreadsheet do it for me, since I don't feel like crunching those numbers for 30 levels across 4 classes.
That takes care of simple bonuses like damage add-ons or boosts to your hit points. For more interesting stuff, I had to get more creative. With the armor that grants damage resistance, I used standard monster damage to estimate how many hit points it would save the wearer over the course of getting beaten down to zero, and treated it as granting that many extra hit points. With the life-draining weapon, I treated it as a combination of increased hit points and bonus damage, then chopped the numbers in half since it only works on bloodied targets.
In principle, this same approach could be used in 3.5. The hardest part would be hammering out what the "baseline characters" ought to look like. After that, it would work the same way... could probably even use the same spreadsheet, with modifications.
[size=-2]*Well, as long as you don't pile on too many "magic item levels." I wouldn't go past 2 levels' worth of items, tops.
**This got tricky with the warpriest. Knight and thief are straightforward, and I built the mage as a pyromancer to keep her simple, but a large part of a warpriest's value to the party comes in the form of healing, which doesn't slot in neatly. I ended up folding healing magic into the warpriest's damage output, at 5/8 value since PCs have about 5/8 the hit points of monsters, and allowing for the fact that healing spells always "hit." I suspect I'm going to have to go back and do those numbers over, though... they scale very unevenly, due to certain spells adding a monster boost when they come online.[/size]