As a caveat, there are some nuances to Wikipedia editing to make sure you're following community standards, which I've tried to lay out in my post. In particular, before investing a lot of time writing a new article, you should check if someone else tried that before and/or if the same content is already covered elsewhere. For example, there have been previous unsuccessful efforts to create an 'Existential risk' Wikipedia article. Those attempts failed in part because relevant content is already covered on the 'Global catastrophic risks' article.

I broadly agree with this and have also previously made a case for Wikipedia editing on the Forum: https://forum.effectivealtruism.org/posts/FebKgHaAymjiETvXd/wikipedia-editing-is-important-tractable-and-neglected

One other relevant resource I'd recommend is Will and Toby's joint keynote speech at the 2016 EA Global conference in San Francisco. It discusses some of the history of EA (focusing on the Oxford community in particular) and some historical precursors: https://youtu.be/VH2LhSod1M4

I enjoyed reading this and would love to see more upbeat and celebratory posts like this. The EA community is very self-critical (which is good!) but we shouldn't lose sight of all the awesome things community members accomplish.

I recently had to make an important and urgent career decision and found it tremendously valuable to speak with several dozen wonderful people about this at EA Global SF. I'm immensely grateful both to the people giving me advice and to CEA for organizing my favorite EA Global yet.

Going very broad, I'd recommend going through the EA Forum Topics Wiki and considering the concepts included there. Similarly, you may look at the posts that make up the EA Handbook and look for suitable concepts there.

See also this recent post: What happens on the average day?

For inspiration, here are some other examples of TEDx talks given by EAs: 
1. Beth Barnes (2015): "Effective Altruism"
2. Gabriella Overödder (2019): "How Using Science Can Radically Increase Your Social Impact"
3. Linh Chi Nguyen (2020): "5 Lessons for choosing an impactful career"

Feel free to add others below that I'm not aware of.

At the risk of self-promotion, I wrote a motivational essay on EA a few years ago, Framing Effective Altruism as Overcoming Indifference. 

Well done! The article receives about 50,000 page views each year, so there are a lot of people out there who benefit from your contribution.