Increasing it as better weapons is increasing the dice size, which follows the weapon size guidelines. Unless you create a new determining factor then that's what you should use. Now, 1d6+1d6/5 would be balanced. That gets you a maximum of 7d6 at level 20. Or you could say that it deals blast damage, and have this just add cold damage at a rate of 1d4/5. I like that one. I don't like your increase chart. First off, it gets weaker from 2d4-1d8 (same max, higher min). Secondly, there is so little difference between 1d6 and 2d6 that at that level, it is totally not worth it.