Kadayi
Banned
Terrible idea.
Imagine, you've almost beaten Smough and Ornstein, no Estus left, the moment of glory is near, then suddenly... Dark spirit xXdarkangel666Xx has invaded!
YOU DIED
Yeah, this would piss me off enormously. Open world, fine, I'll get invaded and fight them off. Boss battles are between me and the boss.
^This.
The one commodity none of us have enough of is time, so the notion of making what will be a challenging enough game even more challenging by throwing in invasions during boss fights (just because...harder!!!) would be a turn off tbh.