Unless this is well balanced, I feel like tier 3's will be op. Here are my suggestions for items to spawn in them.
-Custom Potions (Low spawn chance)
These can be made using Kamyu’s tools (specifically the Potion Chest tool), which can be found here http://www.minecraftforum.net/topic/1133295-custom-spawner-custom-shop-populate-chests-anvil-light-fixer-and-more/. Some ideas that I thought of for these:
Potion of Overdrive: Strength I for 30 seconds, Slowness I for 30 seconds, Hunger I for 30 seconds.
Potion of Protection: Resistance I for 30 seconds, Weakness I for 30 seconds.
Potion of Trolololo(for the lolz): Nausea for 15 seconds
I’ll add more as I come up with them/get suggestions.
-10/15 Arrows (Medium/Low spawn chance)
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Iron Sword (Low/Very Low spawn chance)
Decent weapon, but not an op one.
-Diamond Boots/Helmet (Very Low spawn chance)
They only do half an armor bar more protection, so they won’t be op.
-Cake (Medium spawn chance)
Useful for quickly refilling your hunger while running away, at the cost of low saturation.
-Enchantment Bottles x3 (Low spawn chance)
A stack of 3 bottles almost completely guarantees that you get 1 level.
-Wolf Spawn Egg (Low spawn chance)
If this is added bones would need to be re-added into tier 1’s.
-Vines x 5 (Medium spawn chance)
Can be used to get out of a hole or climb a small ledge.
On the topic of horses: Unless they are somehow limited, they will be OP in my opinion. If it’s possible, I was thinking about some way to make the horse “break” or disappear after a certain amount of time or block traveled.
The chest locations would also need to be randomized to prevent camping/op routes.