Great one! The impact should be inserted by the user/poster, right? The effort is something you as an admin should add to it. Maybe also add a 'cost' valuation to it?
Basically you could filter on an impact/effort/cost ratio to figure out: What is cost-efficient with less effort to build but has a high impact.
Right now a feature that is just a little bit useful to everyone would get a lot more votes than something that is a game-changing mission critical feature to the top 20% of your users. Some systems solve this by allowing people to vote 2-3X for the same feature, but only on a limited number or ratio of features so you can't just say everything is critical to you. As long as there's some way that forces users to be a bit honest about how crucial features really are to them, it can lead to much better feedback than a binary vote for sure!
Maybe a separate "heart" or "star" favorite system (like a tinder super-like) a user could use a limited amount of times could then let you calculate the ratio of voters who favorite it to come up with a sort of impact score. Down the road, perhaps giving users more "super-votes" could even been an engagement/gamification tool to reward more active community members.