An Idea with an AP item

Instead of building a Blasting Wand and Amplifying Tome into a Void Staff , why not have Oblivion Orb be part of the build and take away Blasting Wand or Amplifying Tome away from the build. Then you can maybe add health to Void Staff.

That way, you can get magic penetration through building up into Void Staff.

Or either, create a new item (that gives the passive of magic penetration and) that can build into Void Staff.

(it can possibly gives more ability power than oblivion orb, but it doesn't give health and have a passive similar to Void Staff.

TL;DR Create a new item that only give magic penetration and ability power that helps build into Void Staff(basically like Oblivion Orb but without health) -or- Add Oblivion Orb into Void Staff and have Void Staff gain additional health.