I've noticed that the stat bonuses of fully upgraded equipment (+15) have huge variations, both the type and the amount. This can have considerable impact because usefulness of each stat differ from hero to hero. Also, 5+ star equipment in this game are hard to come by, so its a bit disheartening to see them have not so desirable bonus stats when fully upgraded.
My suggestion is to include a feature in-game to allow us to reset an equipment back to +0. I don't know yet how much it should cost so that it would be fair to everyone, but since you already spend a lot of gold just to raise your equipment to +15, I feel it shouldn't be that costly.
Another suggestion that I have is to allow us to "decline" an upgrade at every +3 intervals (since we get extra bonuses every upgrade of +3). So for example, If I upgrade a main hand weapon from +5 to +6, but I am not satisfied with the bonus, then I have the option to redo the upgrade to +6.