[RPG] Increasing Save DC for new magic items

Proving a negative is impossible, of course, but I'm fairly confident that there are no general guidelines for creating magical items with triggers other than the main four from the question:

• Spell completion
• Spell trigger
• Command word
• Use activated

... plus continuous items.

However, the Contingency spell would probably be useful for the "when X happens, do Y" items.

For pricing such items, the only real rule is to find an existing item of similar power and price your item similarly. The Item Creation Rules have guidelines, and there's some evidence that an item that requires the "right" slot to duplicate a feat costs around 5k (eg., the Gloves of Arrow Snatching, which allow use of the Snatch Arrow feat twice per day), but the rule is to price it based on similarly-powerful items.

I would specifically recommend asking whether a generic character would buy the new item instead of a +X weapon, armor/shield, Stat item, or cloak of resistance: if a generic character is definitely going to buy the item instead of a +2 shield, the price should be higher than the 4k a +2 shield would cost; if it's a toss-up between the new item and a +1 weapon, a price of around 2k is probably about right. This is difficult, especially for new GMs or items that are highly situational (eg., the "self-destruct if a person is too far away from an item" item) or setting-dependent (eg., the auto-healer if divine magic is rare in "this" world).

In my experience, it's generally been best to err slightly on the side of making the item too expensive, then adding some extra treasure if need be.

Remember, too, that "Activating a magic item is a standard action unless the item description indicates otherwise." (Using Magic Items) The rules don't preclude magic items that can be activated as move/swift/free actions, they just don't provide much in the way of guidelines for making them.

That said, it's entirely possible to build most of the items in the question's list using just the Big 5 triggers:

• Self-destruct device if a person forgets some important item somewhere
  • continuous item using Alarm and/or Scrying as a base requirement, plus a sufficiently powerful boom-ey spell
• Magical flashlight
  • use activated or command word item based on several of the Evocation (light) spells, eg., light
• Booby-trapped bags of holding
  • again, adding Alarm to the base requirements could serve to trigger the booby trap, as could simply calling it a variant Bag of Devouring
• Device that casts True Sight on owner when near evil-aligned characters, or perhaps Hide Alignment near good-aligned characters
  • again, a continuous item with Alarm and the appropriate Detect spell (eg., Detect Evil)
• A crossbow that automatically fires a bolt when [given target] is in its cross hairs
  • this is a reasonable interpretation for how True Strike works; an item that allows the wielder to use True Strike with a single attack from a weapon could be said to do this; similarly, Haste grants an extra attack in some situations...
• Necklace that casts a buff when its owner falls prone
  • a constant item with Contingency as part of the creation process could probably do this
• Automatic potion-imbibing devices when its owner is low on HP.
  • again, Contingency could probably be used in creating this

The Magic Item Creation rules are guidelines.

The most important thing to remember when making magic items in Pathfinder is that the rules for making items are guidelines, not hard-and-fast rules. Making new magic items is a very subjective process, and no set of rules is going to give you balanced items each time. The only way to reliably price new magic items is to make new magic items and price them, and see how effective they are in play. Experience is much more important than rules, here.

The base price for command word or use-activated items is for at-will use.

Command-word and use-activated items are, by default, usable as many times per day as you want. There is no limit to the number of times you can use that kind of item in a day.

As an example, the Hand of the Mage. It lets you use mage hand as a standard action, as much as you'd like. It costs 900 gp, which is spell level (.5) times caster level (1) times the command-word constant (1800) for an even 900 gp.

Very few command-word items exist that don't have a per-day limit to their use, because at-will usage of many effects is significantly more powerful than just being able to use it once per day.

Always check for similar effects when pricing at item.

This is the second most important rule of magic item creation. A large number of effects are already represented in the existing magic item rules. If the item that you want to make duplicates the effect of an existing item, then it should cost the same as the existing item.

For your example of an item that lets you cast mage armor you should compare it to existing items that do the same effect, like the Bracers of Armor. The +4 Bracers give you a +4 armor bonus to AC all day long, with no activation needed, and cost 16,000 gp. Your item gives you a +4 armor bonus to AC for 3 hours a day, and needs an activation before it can be used. Clearly, your item should cost less, but exactly how much less is something that's more left to how the DM feels about the item's cost than any hard rule. Personally, I'd go with around 8,000 gp. If you're having about 4 encounters a day, and you have the ability to more-or-less predict half of them, then you're getting about half the benefit that the Bracers of Armor +4 would give you, so I'd charge about half for the item. But that's just how I would run it, and it's more important that you price it in a way that makes sense to you.