I would have treasure troops for souls! I think diamonds would be way too OP, because mythics aren’t that rare anymore after some time
But doing something with souls in general would be cool. And since it is hard to upgrade the hoard of the delves, because of lack of (good) treasure troops, hat would something I would like
Edit: Also have in mind to see the sight of the devs. I think they do a big amount of money through mythics (gems etc). So if you have a free option to farm diamonds, then they will lose this save amount of income, because every player would just farm 24/7 souls and crafting diamonds/mythics.