Currently the way that spells work in 0.2.6 is that spells require a person to gain the level requirement for it, and being the correct class. Could the modding support be put in that a player for instance has two routes:
Level 2 told they can be given either an Icebolt, or a firebolt.
If the player chooses Icebolt, at level 3 the option for an Iceball attack would now be eligible to appear. If they choose Firebolt, then the option for Icebolt doesn't show up anymore, but instead one for Fireball would.
I think it could make for some interesting uses, such as a description in a spell stating ;"The path of the Assassin", giving the hint that similiar abilities of it's nature would then appear for instance. A character deciding to specailise in Summoning could potentially then see more spells in that route, a warrior who focuses on stuns seeing more stuns then possibly if it was modded that way, or seeing more spells involving cleaving.
Could just be a handy thing for modding if it is possible to add at all
ie: certain abilities being able to appear once prerequisites for it (An earlier ability choice) have been met?