I suppose it could be a bit like "elite classes." You have all the base guilds, and then when you reach HM, you have the ability (assuming you have the necessary skill level and pre-requisites) to train something not available to the rest of the guild. It could either be one spell/skill, or one path which might lead to several spells/skills, but disallow spells/skills from other elite paths.

Two obvious paths for an enchanter, for example, might be weapon-crafting and armour-crafting. Or it could be more specialized than that, even...blade artificers, blunt artificers, etc.

Nightblades could have paths that allow for more powerful, varied poisons, or theme-appropriate blade spells, or more illusionism spells that give various combat effects.

It's generally a fun system in other games. Might be fun in Darke.