Due to the addition of Athene's Unholy Grail and Maw of Malmortius, it's much easier for someone to purchase magic resist without sacrificing damage. For this reason, I believe there should be an upgrade on the Haunting Guise, which would make it a viable late game item.
At the moment, HG is an very good choice for an AP caster, provided the enemy team has more than the base 30 magic resist. However, the stats it provides are unimportant when late game comes and Void staff does a better job against high MR champions, while giving 70 AP. For this reason, I believe HG should get the Hexdrinker treatment and have it's magic penetration nerfed.
The new HG could have the following stats:
- + 200 hp
- + 30 AP
- Unique: +10 magic penetration.
Generally, an AP caster needs AP and CDR to maximize it's damage. Therefore the upgraded HG (which I'll name it The Shadowdancer's cloak for the sake of this post) could be the following combination of items.
Haunting guise + Kindlegem = The shadowdancer's cloak (Recipe cost: 800g)
- +300 hp
- +45 AP
- Unique: +10% CDR
- Unique: +20 magic penetration
- Doesn't stack with Haunting guise
- Unique active: The magical cloak protects it's user by giving them an extra 20 armor/MR for 10 seconds (cooldown: 60 seconds). This doesn't stack with other cloaks.
With this item, a caster could avoid building Rylais/RoA and go with this instead if the enemy team has Hexdrinkers, unholy grals, mercs and other stuff our beloved mages hate. This item also doesn't make RoA or Rylai's useless as these give more ap/hp/mana but don't have the CDR and MP instead. This item also gives the option to a caster to build mercs/tabi, instead of Sorcs and still have the 29 MP they had before.
Please feel free to post your opinion on where this item could be implemented on the game and actually be an interesting, balanced choice.
Edit: Tweaked some stats (AP goes up, hp goes down), added active to make the item more unique.
Edit2: Active's armor/mr go up a bit, cooldown goes down.