Hmmm... I was thinking Boneweaver. Perhaps increasing the cost of the farseers and allowing them to do the same?
The reasons are to make larger battles less harsh on command checks for one thing, secondarily, the only real fleet with secondary commanders (Chaos) aren't as worthwhile as CSMs. Unless you want a re-roll, or just really need that 10 points.
In FS every fleet will have access to some kind of 'secondary' commander, like the Gryphon Knight, chaos lords, ork warlords etc.
DE will probably just get Drachon.