Magic Item Compendium introduced us to magic item sets, which as their name implied were thematically-linked sets of magic items that provided bonus benefits for wearing multiple items from that set together. I have been looking in that book for rules/suggestions on how to have this sort of effect on modified/customized items, but I can't seem to find anything explicit on it. Does anyone here have any thoughts on the idea? e.g. what sort of additional costs would it take to add set benefits to items?
The best design is to minimize the impact of “setness”
Sets are a neat idea, but ultimately you run into real design problems if their full power depends much at all on having multiple items in the set. You want flavor and fun from the set bonuses more than you want power.
Ideally, each item on its own is, or very nearly is, worth its cost, and the set is worth the sum total of the components cost. That is, items A, B, C, costing x, y, and z, respectively, should be such that all of the following are true:
There is an obvious paradox here: the various set_bonuses are being added without a corresponding set_cost. The important thing to remember is that there is an inherent opportunity cost for using a complete set: even if B may be worth y, it may not be worth not having B’s slot for some other item. This is true of all items, but sets make it more complicated by having multiple items, and by generally being unique and difficult to move to other slots.
But the only way you can accomplish that is by having the bonuses for multiple items be pretty small; after all, the difference between sets and regular items in terms of the space they take up isn’t that different. So there’s only very little room for extras.
If you do not follow this approach, what ends up happening is either that singular items are overpriced (which sucks for someone who wants to roleplay the collection of the rest of a set), or the full set is overpowered (with the obvious problems that carries, plus possible dilution of sets’ “specialness”).
I've once made a whole quest around a set, so the meaning of it (hence the cost/balance) was something else.
The set was also partly a McGuffin:
"The legend goes that only with the Light-Powered-Sword held with the Light-Powered-Gloves one may break the Wall-of-Shadow, all around the Lichs-Dark-Castle"
KRyan pointed out something very nice about sets: In the end, the cool thing is to have the whole set.
I would recommend to balance the cost thinking about the full set but, of course, do take in account the hard adventure to obtain the set pieces as part of the cost. As the players have to work for the items, rather than just buying them, it becomes more balanced.