My idea to solve both spam and vote buying is to encourage those like @blocktrades and @minnowbooster, even @berniesanders if he needs a new direction, to change their business model. Instead, delegate to trusted curation teams like OCD, Steemstem, steemiteducation, some of the country-based ones and so forth, who will then do the quality curating with that SP, thus boosting the minnows and reducing spam ability/incentive
To get an ROI, they could offer a premium where any user can send a small payment to prioritize viewing of their posts (but not guaranteeing an upvote). There are enough people in these teams to cover most tags and can always recruit more.
Convincing a few whales would be a far easier solution than changing the framework of steemit via hardforks and the like, and they'd still profit if done right