I do not know if it has been already pointed out in another post and if so, I apologize.
This post is intended to be a suggestion for developers:

For a lev. 30 who would like to change strategy to heat it's impossible to obtain an appropriate legendary harpoon, if you have not been lucky to obtain that lev. 25 previously, once you become lev 30 you can not get more lev. 25 heat harpoon because item box allow you to obtain only from 28 to 30.

This is a major limitation to the variety of game play.

I propose two solutions:

1) a new lev.30 heat harpoon
2) transform the existing lev. 25 heat harpoon to a lev. 28, so you can get it from item box

what do you think about?

They should make a heat harpoon for level 30 and a physical harpoon for level 30