In forgotten Gods there is coming ability to reforge one unique to another and one set piece to another, just like in Diablo 3.
What if you add one more option to the mix.
Ability to upgrade item tier.
Reforging nomal item to legendary to mythic.
Or even ability to downgrading tier.
I think that would cheapen the rarity of mythicals, and trivialize the ability to complete set pieces. Just my 2 cents though.
Of course we are not talking about cheap way to upgrade tier.
The cost of upgrade should be comparable to crafting cost of mythic items.
I’m afraid you’re not quite right in your initial premise. We’ll be able to turn one set item into a random item from another set of about the same level or one set item into another item of the same set. Not legendary -> random legendary. You’ll still do that (and the equivalent of most other things you mention) via blowing up epics & legendaries and taking the crafting materials to Etram Fald or Bella Fald.
Would be nice to be able to upgrade non-mythical items to mythical. Non-mythical item + some solid amount of rare crafting materials -> mythical. But as that would be kind of nice for some builds, crafted items lose the craft bonuses and all items have a probability of reducing some stats by an amount. For example, converting Leviathan with +7% armor to Mythical would make it mythical, would lose its armor bonus and re-roll all stats, now it has e.g. 20% chance of reducing a stat due to a conversion “error”, and we were unlucky and we lost 2% of our physique. We could then repair it (by paying rare crafting materials) to remove a percentage (or fully remove) the lost stat.
I don’t know if that would be actually cool, but it sounds so to me. 