I have previously run into issues with homebrew items throwing off the balance of my game, and I'm trying to anticipate any issues I might run into by giving my players certain items. My players enjoy receiving items specifically designed for them, and I like to enhance their game beyond the items in the books. However, I would like to be aware in creating limits for these items. What I've noticed is that there are two main categories of such items.
Encouraging What They're Already Doing
For example, I have a ranger who switches between his bow and his greatsword, so I would design an item that encourages him to swap weapons, by giving him a bonus to him the turn that he switches from a ranged weapon to a melee weapon. This encourages him to do more of what he's already doing.
Encouraging New Options
This is the one I have more trouble with. For another example, I have a Spirit Shaman, adapted from D&D 3.5's Complete Divine sourcebook. To fuel her magical item crafting (using a homebrewed crafting system), she collects the essences of the dead that she sends to the Spirit Plane. I planned on giving this character a robe that would allow her to use these essences to enhance her magic, especially against a certain subset of creatures that are prominently featured in the game. In addition, this robe would allow her to use a part of her own spirit essence (cast from a percentage of hit points) to use this ability. It ties in with the character's self-destructive tendencies.) My problem with this is I don't know how using this would disrupt character balance. I would like to design items like this for all of my characters, as I feel it enhances options, but I would like to also keep tabs on them and design inherent limits to them.
How can I design items that either encourage new options, or further the options they already have, without either of these becoming the dominant strategy?