I don't think this is how it will go, but..
I think it would be interesting if the Scholar stone could be used for EITHER Conj or Thaum and that choice would define which way the Scholar leans. For instance, you'd get the base spells of either the healer or nuker class, and then the Scholar specific spells would enhance whichever role you chose to equip the soul stone on. So you could be a Scholar with Stone/Aero/Heals/etc + Scholar specific spells, or you could be a scholar with Fire/Thunder/etc + Scholar specific spells.
I can't see them attaching it to Arcanist, it just seems too wonky, my money is still on a new class altogether.