Designing by Type
Ammunition
Armor and Weapons
The specific item should cost more than the base grade armor or weapon would with just basic weapon and armor upgrades, but you can often discount the cost of the additional components significantly as part of the specific item's special niche. Be careful about specific armor or weapons that include weapon upgrades in addition to unique specific abilities. If you discount the item, you might end up with an item significantly superior to one built using the normal weapon upgrades system. That's not always bad since it's still giving up customization for power, and this can be appropriate if the item has an important place in your story. Just make sure the difference isn't too drastic. If you just want to create armor or a weapon with upgrades and no extra special abilities, you can do so. The Price of such an item is the sum of all the upgrades' Prices, and its level is that of the highest-level weapon upgrade on the item (if higher than the grade of the weapon).
Runes & Upgrades
Augmentations
Held Items
Remember that held items are more challenging for martial characters to use, compared to spellcasters or hands-free characters that specialize in unarmed attacks. A soldier might have to give up a two-handed weapon to use a held item, and so is less likely to use one. This means you might want to design held items specifically for non-martial characters, or have them be items a martial character uses outside of combat.
Medical Items
Spell Ampoules
Spell Chips
Spell Gems
Shields
Upgrades
While the upgrades in Player Core are exclusively for armor and weapons, you can design bespoke upgrades that apply to other items when necessary. Upgrades that go on items other than weapons and armor should define that the item can only use one upgrade, as only armor and weapons can get additional upgrade slots. In most cases, you're better off designing a higher grade for the item or designing a bespoke item that functions alongside other specific equipment, such as the reusable grenade shell.
Worn Items
Apex items are always at least level 17 and should have unique abilities on top of their bonus, just like other items.