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?

This sounds a bit complicated. If I have 2 spells on the same level and I don't pick one I can pick it on next level, yes? If I have 2 of such spells on every level I don't have enough slots on level 5. :)

I like the system of single path for a profession and possibility to have a different ability from other professions. I would like to grow it by adding more unique professions as "paths" you mentioned. I'm not sure how to integrate your idea into the game without breaking this.

So, an unseen skill tree? I think that would be pretty counter-intuitive. One part of the suggestion that I do like is that if you don't pick a skill at a certain level, it can appear again later. Having a random selection of skills to pick from is great, but it bothers me that I can only pick up certain skills at a specific level, especially since the greater part of the game is left with no new abilities to reward advancing the character level. I would love to see a progression system that recycles unpicked abilities from lower levels, so that I have more long-term control over my character builds.

I realize the randomness of the ability selection contributes to strategic challenges and excitement when you get the ability you want, but once you're in the character levels where there are no abilities left to pick from you have already overcome those challenges, and it would be nice to be rewarded for that.