A few things.
I am a firm believer people come to chans through their own effort, and self sort into what interests them. Chasing after other board users is almost a complete waste of time for more niche boards. We need to advertise, but we need to pull from off-chan, outside groups. Drop some flyers around the student union lounge, etc.
We could raid a professional philosophy board. Would not work in practice unless, in addition to the usual starting shit just for the fuck of it, we link the idea of: "show us why philosophy on an image-board and do us some outreach" thing. This might make them willing to stick around.
Change the board to be more inclusive. Umbrella the board with the humanities and sociology, not strictly philosophy.
Find a way to link us up with anime and cartoons.
Philosophy on western image-boards remains focused on politics. For good or for ill, that's what is popular. From the perspective of these users there is no need for more expansive theoreticals.
>Are you autistic?
4U