For mystics, David Renfield provides bonus but many investigators avoid using him due to doom. Now, Elle helps to solve that problem. With Renfield + Elle, you can get +1 bonus as well as 1 free resource for each round; notice that you do not choose to add doom onto Renfield when triggering his ability.
However, some problems remain. We need Charisma, so that true xp cost of this combo is not 2xp but 5xp (or 7xp for 2 Elle in the deck). In addition, we give up the great ally, Arcane Initiate, or need additional 3xp. Finally, the overall cost of Renfield + Elle is 5 resource, which is high even if we could earn 1 resource per turn. Of course, it is good chance that we revisit David Renfield without any fear about placing a doom.
I write the above review before the least taboo was revealed and now that combo is 13xp. Now, that's too expansive combo IMO.