So, for starters, there needs to be a reason for the PCs to be together and investigating various weirdnesses; there needs a coherent setting in terms of what's "normal" for the PCs; and there needs to be a progression of events that doesn't railroad, but gives incentive for the characters to pursue the plot-line.
You all meet in a speakeasy: not
One of the things I feel is important for trying to make this a campaign as opposed to a short adventure is there needs to be a sustainable rationale for the PCs to not only investigate things, but to continue to associate with one another. The hook I'm considering is to have the party all be either students or junior staff at a college or university. To further "push" them together, I'm thinking in terms of a small college, probably somewhat rural. Back in the 1920s, such a locale could be an island of learning in a backwater town or similar. These rural farming towns and the like are classic backdrops for Lovecraftian horror. The campus gives the characters some resources for research, etc. but its library and pool of scholars is limited. Plus, the daily routine of student life provides a backdrop of normalcy.
As far as a motivation to investigate, the device I'm considering is for each PC to be a recipient of a "research fellowship" from an academic society. The fellowships cover tuition, etc. and provide a small stipend for living expenses. The catch, however, is that the society periodically may assign "field work" projects to its beneficiaries. This may be simple research in the library, cataloging materials, or actually going out and tracking down clues to greater mysteries. None of this exempts the PCs from their responsibilities as students or staff, though.