One thing I don't get about the SMT universe is how "unique" demons with personalities and histories fit into the whole fusion thing. Like Lucifer is often off limits because how important he is to the game's world, but in theory, you could just fuse him right (in some games I'm sure you can)? So what happens then?
I don't think the main games ever went into explaining that very well. IIRC, basically, "fusion" a lot of the time, especially regarding stronger unique demons, seems to be actually closer to "summoning". The demon brought forth receives the knowledge of the previous demons (explaining the inherited abilities) but otherwise is its own thing.
The characters talk once about that in Devil Survivor, and SMTIV's fusion mechanics and demon dialogue seem to support that too. When you fuse demons to make the Minotaur, it's always Akira's minotaur, the one that you faced as a boss and got fusion data for him after his defeat, who even references your previous encounter with him when he appears, not a random newly created minotaur.
When you're just fusing a generic demon, although you're probably actually creating a new demon, and they keep their memories, their personality still instantly changes to match their new race.
Lucifer just dies forever and gets turned into a Jack Frost if you fuse badly enough? Can you make your own Lucifer? Is it that the Lucifer? Can there be two Lucifers? That's just looking at a dude with some degree of importance.
And then, if you think about it, God is just another demon right? So can you fuse God into a shitty monster too?
Like I said above, "fusing" strong demons most of the time seems more like sacrificing some demons to summon a more powerful one. You aren't really actually fusing them together. In the same way, fusing those strong demons seems to just make them "leave" while somehow helping with the fusion process.
This is Lucifer's quote after being fused in Devil Survivor:
"My name is Lucifer the Tyrant, bringer of light. I've awaited this day, my children..."
In the mainline games, you can never have more than one demon of the same species in your party. In the Devil Survivor games, there's a mechanic where certain demons are "unique" and you couldn't have more than one of them among your troops, while others are generic and you can have multiple ones, which just further suggests that the seemingly "unique" demons are just summoned by the fusion.