Since steemit already released the promotion feature, I recommend changing this:

Give users more things (like 'promoting') that they can spend their Steem/SDB on via the UI (see post from @intelliguy)

To this:

Give users a virtual goods / feature rich STEEM store to buy things from (like avatars, extra features, etc) and the automated shopkeeper burns the profits, which takes steem out of circulation and helps raise the market value of steem.