Coin can't be both the money AND mana item.

I believe to make spellthief's more relevant, riot could strip coin of its superb mana sustain and move it to spellthiefs. Like giving it mana restoration in addition to gold when attacking enemy champions+turrets. If it is supposed to support poke/AP supports this will help them do their job better and give spelltheifs a more distinguished role. Coin can still be used for passive lanes that just want to get lots of gold. Maybe even giving it some health regen in exchange for giving up mana. What do you think?