What if, Rabadon had LESS base AP but a higher % Coeffecient
Thus making it much stronger as a 2nd or 3rd item, but slightly weaker as a rush item. Slightly reducing the overwhelming burst of mages in the early game but improving their burst for the mid and especially late game. Helping them scale better.
Something like +100 AP +40% AP
With 0 AP and buying Rabadon Old - 156 New 140
With 100 AP Old - 286 New - 280
With 200 AP Old - 416 New - 420
etc