I think it would be awesome to create new emojis that visually represent the weightings. Like emojis that are actually +1, +3, +5, +8 icons. I think it would be easier for newcomers to learn a new emoji system layered on top of the way they already use emojis, rather than relearning how to use emojis they already use.
Some existing discussion of this: When to ❤ something? - #3 by wchargin