Reward the ecosystem to bring in 10million accounts
Have a compulsory (can be interpreted many ways) referral link for signup (in blockchain), create an internal top 100 (weekly, monthly, quarterly) recruiter ranking of new signup and also top 100 recruiter new account's total 1week, 1month, 1quarter accumulated highest earning (or invested SP) / per account to reward top talent & investor recruitment.
Then payout 1% (really it's kinda like a blockproducer finders' fee) of all BP at current 8.85% annual inflation to be shared by these top 100x6 💯 recruiters, or about 3800+ Steem paid out to over >100 people (really 100-600 individual, but most will be duplicate so more like 150 is an average estimate), that's 100 Steem/month bounty for finding new signups per top recruiter (honestly, that's slightly better payout than a Witness at Rank 90 today)
Then watch how suddenly it's in fashion for every Steemian and their dog to recruit every living soul with a phone number and email to signup via Steemit! And even better signup folks who might have some real talent to actually produce proof of brains to get double payout!
Update: this was written 2am in the morning after taking some liberties with alcoholic awesomeness, please scroll below and see the even better idea proposed by @roelandp below
also sorry @themarkymark, i did not fully read your post i was told about @trumpman's funny reply and I originally came to read it to get some entertainment an ended up commenting, not here to win any prize am sure many more suggessions are way more worthy