** Please note that this is untested, and may turn out to be too much, and I may alter it later. However, I may also make these powers stronger, if they don't prove to level the playing field between barbarians and casters, as intended. **
As my game is high-level and magic-heavy, and this sort of game tends to make Barbarian characters somewhat useless, I've added the following to make them more fun to play.
Saving Throws - any barbarian character that fails a saving throw, even on a 1, can choose to draw deep from within their reserves of life and reroll the save. This cannot happen more than once a day.
Magic resistance - Barbarians become gradually more resistant to magic as they level. As noted in Unearthed Arcana, this grants them much better saves vs. magic, but they also gradually become resistant to magic, and their very presence can dispel magic, in a 1' radius per level of the barbarian. This works like this:
(Barbarian level - spell level) x 5 = % chance that a non-permanent spell is dispelled. This is not a controllable effect. It will only need to be checked once for any given spell for any given barbarian, unless that individual increases in level. (Note that 'any given spell' means a specific casting of a spell by a specific caster.)
This power is modified as follows, though:
1. Familiarity breeds trust, so for each month that a barbarian spends around a particular caster, this radius will shrink by one foot, until it is only effective on touch. Note that a 'trusted caster' who does some harm to the barbarian will cause this reduction in radius to instantly vanish, presuming the barbarian knows who did the harm.
2. Barbarians who use magic items are considered to be lower level, although one armor item worn, one shield, and two weapons (counting bow + arrows as the like as one weapon) will not affect this; beyond that, any additional weapons or armor carried will reduce the barbarian's effective level by 1 each; every five potions will reduce the level by 1; any scroll will reduce the level by 1 each; any miscellaneous magic, ring, rod/staff/wand will do the same.
3. Cleric and druid spells are considered to be 5 levels higher, reflecting the greater innate ability barbarians have to be able to associate with clerics and druids.
4. Caster level is not a factor.
Missile weapons - Barbarians have a greatly increased chance to not be hit by missile weapons if they wield a shield. Armor class vs. missile weapons is increased by 1 point per level against missile weapons.