Examples of pre-launch traffic building?

I think about marketing a brand new product in 2 "buckets":

• "launch" marketing activities — Product Hunt, beta users, Twitter, share it in communities, etc.
• Traffic channels — Building up one or two key traffic channels that will sustainably send potential customers, increasingly, throughout its first year and beyond.

As a product person / creator, I know my way around #1, the "launch" marketing.

My question is on #2 — building a traffic channel, early.

What are some ways we can get a head start on growing one or two of those key traffic channels—before a new product has been launched, perhaps even before it has been "revealed" to many people. Naturally, these channels could help with launch, but more importantly, this is what will sustain the product and grow its customer-base through its first year.

A few ideas come to mind:

• Start SEO & content on core topics for this problem/space
• Start a weekly newsletter curating content for this space
• Start a podcast for this space

(And now I'm out of ideas...)

Do you know of examples of businesses that executed marketing really well, starting from before they launched (or very early on)?

If you're hired as a marketer by an early-stage product, what kinds of strategies would you want to execute early?