Wouldn't it be great if Kat fell pregnant when she was raped by her demonic father. Kat had the baby, yet the baby was thought to have died during childbrith. Secretly the baby was taken by the demons.
Fast foward to the events of DmC 2. The half demon - human child of Kat's make's an appearance as Vergil's ally. Willing to do anything to satisfy her master Vergil.
A human - demon hybrid is no match for a Nephilim, yet Vergil wants to punish Kat for choosing Dante over him. So he forces Kat to make the same decision. Kill her child or save her child and betray Dante.
Although I feel (if there is another one), the angels need to be a prominent force. Perhaps they would try to trigger an appololypse to cleanse the world of demons, and they offer Dante a chance to survive, but all the humans would be killed as a consequence. In the end I can see Dante and Vergil joining the demons in an all out war for survival to save themselves and the human race.