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So, all we need is the ui that works as described?
Enter username desired, get owner key, input deposit, and viola, new hivizen?
I guess we would need some additional data, number of expected transactions per day, benefits of full accounts, etc,...

That's right

Any idea what that would take in dev time?
Do I need to make the proposal, or what?

Not very informed about Hive UI development so I can't comment.