[Item Conept] I'm going to make this work !

Forget this.
I got the solution :
There just doesn't have to be a "core item" from which they all build.
The theme just has to be present. Problem solved !
So here we go :

(NEW) Memento of the Beast

• Recipe : Long Sword + 440 Gold = 800 Gold Total
• +15 Attack Damage
• UNIQUE Passive : Basic attacks deal 10 magic damage over 3 seconds on-hit.
• UNIQUE Passive : +5% Cooldown Reduction

^Image would be a big red scale with a big claw and a small red stone (the red buff animation stone) with sigils embedded.
Champions I would buy this on : I don't know ... AD peoples ? Anyone who wants to build the other items this builds into.

(NEW) Memento of Nature's Guardian

• Recipe : Ruby Crystal + 480 Gold = 880 Gold total
• +200 Health
• UNIQUE Passive : Gain Mana Regen equal to 0,5% of your maximum Mana per 5 seconds, if you haven't received damage in the last 6 seconds (excluding minion damage).
• UNIQUE Passive : Killing a monster restores 6 Mana.

^Image would be a big mossy stone, with a hexagon blue crystal, in which a sigil is embedded. Next to the mossy stone lies a small blue stone (the blue buff animation stone) with sigils embedded.
Champions I would build this on : , as a core item. She can use her long range to use the Mana Regen efficiently, she kills the Wraith camp when she has nothing to do and it would build into the current . Similar to that , , , and .

(NEW) Memento of the Undead

• Recipe : Amplifying Tome + 405 Gold = 840 Gold
• +25 Ability Power
• UNIQUE Passive : Your spells restore 6% of the damage dealt (one third amount for AoE spells) against minions and champions as health.
• UNIQUE Passive : Grants 1 Ability Power per large monster kill (max : 10 stacks).

^Image would be a big, black crystal (two pyramids stuck together).
*Champions I would build this on : * , ,

The Mementos of Nature's Guardian and the Undead would replace Chalice of Harmony and Hextech Revolver.
The solution I found to the problems was splitting the item into two passives, one for monsters, one for minions and champions.
That way I can keep the theme of jungle themed items and make them build into the spirit line, but also build them into other items. I simply use the one passive for the spirit line and the other for other items.

Here's the rest :

(NEW NAME) Spirit of the Greater Wolf

• Recipe : Spirit Stone + Memento of the Beast + 500 Gold = 2000 Gold Total
• Item remains otherwise unchanged.

(NEW NAME) Spirit of the Stone Guardian

• Recipe : Spirit Stone + Memento of Nature's Guardian + 420 Gold = 2000 Gold
• Item remains otherwise unchanged.

Spirit of the Spectral Wraith

• Recipe : Spirit Stone + Memento of the Undead + 460 Gold = 2000 Gold
• Item remains otherwise unchanged, stacks are lost.

(NEW) Armor of the Elder Lizard

• Recipe : Memento of the Beast + Giant's Belt + Ruby Crystal + 700 Gold = 2900 Gold
• +600 Health
• +25 Attack Damage
• +10% Cooldown Reduction
• UNIQUE Passive : Basic attacks apply a 3 second debuff, that slows for 24%(12% for ranged champions), falling of over the duration, and deals 30 magic damage over the duration.

This item replaces Frost Hammer. The slow is weak, but it deals bonus damage and lasts longer, it has a lower cost and it gives 10% Cooldown Reduction. So overall the item is better for recourse-less champions, that want an extra slow. Such as , , , ,,, , , , , and , but also many, many other champions, who don't need the Mana from or just rely on basic attacks more than spells.

(NEW) Ancient Grail

• Recipe : Memento of Nature's Guardian + Forbidden Idol + Amplifying Tome + 535 Gold = 2600 Gold Total
• +300 Health
• +40 Ability Power
• +15 Mana Regen per 5 seconds
• +20% Cooldown Reduction
• UNIQUE Passive : Gain Mana Regen equal to 0,5% of your maximum Mana per 5 seconds. Additionally, restore 12% of your maximum Mana on kill or assist.

^Image is a moss-covered stone grail with for hexagon blue crystal embedded.
This item would replace
The Health is equivalent to the magic resistance in cost efficiency, but it also helps against AD champions, the ability power was nerfed to underline the items role = offer sustain and survivability for mages.
The sustain was linked to another stat - mana - to make the item factor dependent, and not just give straight out sustain.
Overall, it wasn't changed much.

Will of the Ancients

• Recipe : Memento of the Undead + Fiendish Codex + 840 Gold = 2500 Gold Total
• +80 Ability Power
• +10% Cooldown Reduction
• UNIQUE Passive : +20% Spellvamp

(NEW NAME) Shadow and Light

• Memento of the Undead + Bilge Water Cutlass + 1160 Gold = 3400 Gold Total
• Item remains otherwise unchanged.

^Image would be Lucians's pistols crossed. Replaces ->

(NEW) Scale of the Voidworm

• **Recipe : ** Memento of the Beast + Memento of Nature's Guardian + 920 Gold = 2600 Gold Total
• +300 Health
• +25 Attack Damage
• +10% Cooldown Reduction
• UNIQUE Passive : Gain Mana Regen equal to 0,5% of your maximum Mana per 5 seconds.
• UNIQUE Passive : Basic attacks apply a 3 seconds debuff that deals 14(+2x level) magic damage and reduces the enemies current mana by 6(+1% of your maximum mana) over the duration.

^Image would be a big, purple scale.
It's a type of anti-caster item for bruisers. While it doesn't do much for assassins, prolonged fights can burn away the enemies mana quite quickly.
Maybe some tanks could buy it, since it doesn't require all too much attack speed, it's better if you can stick to your target, gives Health and Cooldown Reduction, free damage and sustain and doesn't cost too much.
I could image it on .

And so, all 3 items build into 3 items total =)
Thanks for reading and please leave feedback in the comments.