It's an interesting idea, and maybe I don't have a firm enough grasp of the workings of Steem, but it would undermine the purpose of Steem Power and powering up. If you're upvoting something, it's in your best interest to throw your full weight behind that "promotion". Upvoting is basically saying "I think this content is popular, valuable, and will continue to be upvoted as more people see it". To add a slider is to lose the meaning of the upvote.