If something like this already exists, please let me know.
Simply a roleplay thing. Eg: I'm currently playing an alchemist/necromage who lost someone important to the draugr, and is now dedicated to exorcising them wherever they are found. I imagine sun magic isn't a lost art (possibly slightly rarer if anything) so it stands to reason the Dawnguard wouldn't be the only ones with access to it. I can easily just spawn the spell tomes and drop the corresponding gold cost for RP purposes, it's not a big deal. But still it would be cool to see Collette selling them.