After the lengthy explanation of current issues with Fortitude, Reflex, and Will progression, I think we are ready to look at the crunchy bits. The house rule I propose for the non-AC defenses is not, in fact, a single rule, but a set of three different modifications: the introduction of masterwork neck slot items, the creation of a new type of magic item that ensures a minimum ability modifier to defenses without costing any additional item slot, and the ban of a series of feats.
1- The masterwork necks
The first step of the patch is to allow the use of masterwork neck slot items. Following the same principle as masterwork armor (see PHB, p.212), a masterwork neck grants a higher bonus to Fortitude, Reflex and Will than a non-masterwork neck. The type and level of a magic neck item determines if it can be masterwork, as detailed in the table below. The cost of a masterwork neck is included in the cost of the magic item, so higher-level characters should seek out masterwork necks instead of normal magic necks.
MAGIC NECK SLOT ITEMS
Masterwork necks are a functional replacement for defense-enhancing feats, providing equivalent bonuses without any cost for the PCs. As such, they fix, by themselves the problems associated with good defenses (as defined in the previous article). However, an additional measure is needed in order to patch bad defenses.
2- The Lucky Badges
A player's bad defenses suffer from lack of progression in ability modifiers. In order to compensate for that, you can allow players the use of the Badge of the Lucky Hero magic item.
If you are using this house rule, a level-appropiate Badge of the Lucky Hero is an item that every Player Character needs and is assumed to have, much like PCs in the basic rules need and are assumed to have level-appropiate magic Weapons/Implements, Armor, and Neck items. DMs will have to assign additional treasure parcels to account for that: for a normal 5-player party, an additional magic item of the PCs' level for each level would be enough.
Alternately, you can have all characters in a party earn a Badge of an appropiate level upon completion of a major quest, instead of granting additional parcels.
Lucky Badges ensure that a character's defenses don't fall below a minimum threshold regardless of having low ability scores. This threshold is set so that monsters of the same level will hit these defenses roughly 70% of the times - a value noticeably worse than that of any defense affected by primary or secondary abilities, but high enough for enemies to occasionally miss.
3- The ban
While using this rules patch, the following feats should be disallowed, since their bonus is effectively given for free:
The following feats are still permited, since they allow players to customize their defenses through modest bonuses, and are neither dangerously strong nor required for most builds:4- Changing your defense values in Character Builder
Unlike attack bonuses, your defenses can be modified in a Character Builder sheet. In order to do that, you only need to double click on the defense score in the character sheet, and type the desired value.