A cap is an interesting idea.

And I do agree with @shaka that valuation is a very subjective thing and sometimes it's hard to also justify why a picture of a street corner or a picture of what one had last night cannot be worth $10.

I ain't well versed in the steem witness yet so I can't say anything about that. It would be of my opinion to keep the structure as simple as possible for the time being.

Sometimes a user may have devoted a lot of energy before to build up a lot of SP or they had the foresight to buy up lots of SP when it was trading lower. They also need to be rewarded for that. So, a SQUARE ROOT of an upvote may serve as a better idea.

I plan to come up with some statistics later in my posts and look at the relationship of STEEM PRICE vs. steem newbies (minnows) upvote success rate. Stay tuned.