I would really love to play with some of my friends in higher levels.. but most of them tell me "Sorry bro, no ancient weapon yet, we won't get far" or "Sorry bro, I'm playing support now because I still have no ancient Krider/Sparker/Dart/etc" I know so many good players (some not even my friends) that if only they had ancient weapons, they could be the top of their class.
From what I've seen this functionality is not currently present. However, it looks like you might be able to upgrade rares into non-ancient Legendaries.
It seems that you can reforge an item. All you really need is to find an Ancient Weapon with a decent that you want, and you imbue the affix that you really need through the cube.
Kanai cube fixes that problem indirectly. You won't need to find that ultra rare ancient legendary. You just need to absorb the orange text and use it as passive and incorporate it in your own build. This game has so many possibilities now. Can't wait to mix and match.