Players who start upgrading the items after the first stage of simplification will be introduced to Endo, which can be created from duplicate mods or dropped where Fusion Cores would previously be found. Endo can be invested in an item to move it toward the next rank, with whole-rank increases remaining intact but all partial upgrades being removed. The core upgrade loop will remain the same, but it should be much easier for players to understand what they’re upgrading and what the end result will look like.
Warframe; it’s how you upgrade your mods, it’s why you want to keep various mods around, it’s a major part of the game. It’s also more confusing than it needs to be, according to the developers. Thus, Fusion is being simplified in two stages. The first stage will make Fusion much simpler and straightforward, while the second stage will be more about lore and collection improvement.
Source: Official Site; thanks to ThatLanteshGuy for the tip!