Continuing my post above... tl;dr : Remove fusion costs.
The fusion system is fine WITHOUT fusion costs.
You have to account for having the items themselves to fuse, which costs coins if you'd like to purchase boxes. Then you have to account for the transformation costs. Then the power drain, where a full powered item will give a much smaller amount of power when fused to another item. Lastly, you will need to adapt to the metagame, so you will end up repeatedly fusing your current items into other items.
Alternative suggestion: Remove fusion costs for the grace period, so veteran players will have a chance to fuse away all their 5000-10000 items. Even then, the default fusion cost is too high, so it should be MUCH lowered after the grace period. It needs to be divided by like 10, or at least 5. I mean for real, not monkey math.
I suggest just removing fusion costs totally. (not transformation costs)
Either way, something has to be done about this, the alternative suggestion I posted above would be alright, IF you guys still think "fusion costs" should be a thing.
Also, this is my gripe on the game currently. I am sure many other players have other issues they take offense to, such as all their hard-earned items being useless now. They can't even fuse them, because of... once again... the fusion costs.