I hope we'll be able to add our own schools by modding. Dark Magic and Divine Magic are in for me.
that would only be a "naming" issue.
ie, if you are not able to create new skills, additional schools are rather pointless.
to my knowledge skills can only be altered (ie eg different range, more/less damage, combination of existing effects to a new spell etc.)
completely new skills (ie like shapechange in DOS1) are not possible to mod - at least, i could not figure out how.
i think @forktong was responsible for skills/spells in DOS1 and i guess he will play a major role in this regard in DOS2 as well. maybe he could (oc only if his time allows for it

) comment on the future of modablility of skills/spells?