The single most useful thing you can do to help Farcaster grow is cast 1 thing a day in a channel. Especially non-crypto related.

A link to a blog post in /engineering 

A song you think /electronic would find interesting. 

A long cast about your preferences for Japanese knives in /food.

This is from users' POV.

Tho from builders standpoint, IMHO, building topic specific apps or clients based on channels would be useful for vertical growth, adding value to the whole FC simultaneously.

There are barriers (like cast size) atm, but the potential is still huge.

For example: One can define channel specific patterns (like hashtags: #, questions: ?, etc) & render it in an special way in the client.

p.s.: as I'm writing this cast, the cast length limit circle down there extended the cast limit telling "First 320 chars will...."! Seems I wasn't active enough on Warpcast to remain in the loop of changes. Great feature.