> ? retroactive funding - attendees pay after based on value received
Bad idea. Why? Because you shouldn't slide into "non-profit" crap if you want to make such an event successful (not for profit, but for the sake of ethics).
> ? speaker lottery - submit talk proposals, random selection prevents "known name" bias and commercial gaming
Bad idea. Why? A simple example: I see Speaker_1 and a topic in the list of speakers, and it's personally very important to me to hear specifically him, but then it turns out that Speaker_1 won't be speaking. I want to know in advance which speakers and topics will be discussed, and I'm gladly ready to pay for that.
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I like everything else said above in your post.
For the first event, you could consider the idea of renting a villa in some village in Spain, or several villas next to each other, and making it a multi-week event, so that people don't just listen to "blah-blah about decentralization" but also present their projects, live together, hang out together, go hiking, and so on. A successful example ->
@Sovereign Engineering.