DMC and DMC3 were ahead of their time in terms of boss encounters in an action game. Agni and Rudra wasn't really matched until Dark Souls did that double boss fight.
Think is that DMC4 did not evolve from its PS2 design. Same level design, enemy encounters and boss fights. Where as now we have PS4 level design and boss fights. The bosses in Bloodborne have like 15 moves in their first phase and then another dozen in their second state. That's crazy when you think about it.
So right now DMC is two generations behind in terms of design. That's why I said so many times that it's not sufficient that they just make DMC4.5.
They have to rethink a lot of aspects of the series. The basic controls, the camera, the lock on mechanic, the Dodge system, the styles system, the level design, enemy encounters, lore, level design, boss fights, extra features, online functionality, animation and physics, the writing... all the while keeping it faithful to the series.
It's a really hard task.