I Agree that need some feedback when starting working thinking that is okey but when you launch it will always see some issues etc. Next plan I think during free time create Web app instead Android app more wide use and less bugs than native development.
First Idea in my mind:
1. The Horadric cube with some filter and recipe filter With chance to add to favorites and later use custom filter from favorites or similar.
Amulet, ring or Crafter items: Blood, Caster etc. will be more interesting when you able to hit click transmute and see output with affixes.
!! Not sure about freely picking materials like gems, items, runes and drag or click to add to cube I think is add a bit control but more complex system for new players and there is no reason to do it
2. Add some more tools not sure about tools which already in development from other devs. Because if you already played diablo 2 basically all builds in mind and starter classes is by default picked if you more oriented farm alone