I think you're asking two slightly different questions. The first is "How can people best accelerate bitcoin adoption?" and I think the answer is education and representation. Talk to as many people in your family, friend and peer groups as you can. Educate them about why bitcoin matters, how to safely buy and protect bitcoin, &c. The motto of bitcoiners is "Verify, don't trust" yet most bitcoiners got to where they are by trusting some friend or family member that helped them understand and onboard into bitcoin -- at least initially. Be this person for your friends and family!

The second question you're asking is "What would you build now?" -- not everyone is a builder of things, so this question has a narrower audience, but it's just as important. I tend to start technical companies at the early stages of the adoption curve of some technology. Bitcoin is still early overall, but a lot of areas have gotten mature (e.g. exchanges). I'd probably be working on Lightning, as it strikes me as both extremely early as well as poised for rapid growth!