I'd love a mod which did this as well. The exploring nature of the quest does seem more in line with bards than thieves. I know some of the gems are in NPCs homes, but they're tagged as take not steal, so it's not technically thievery. Or failing a mod which changed it to a Bard's College quest, something which let's us get the quest from Vex without joining the Guild. Realistically we should be able to approach the Thieves Guild as a client rather than being forced to join for the quest, since we pick up the unusual gem ourselves and need to find an appraiser, not get sent by Vex to find the gems. Which gives me another thought. It would be interesting to have a mod that allows us to hire the Thieves or Companions for jobs if we've not joined them. I don't know how it would be done or if it would be feasible, but it would be interesting for immersion's sake if we could hire the companions to clear out caves or the Thieves to steal an item from an NPCs house, or be able to perform the Black Sacrament to send the Dark Brotherhood after someone.