So had been the idea of a sibling rivalry where one twin is evil (DMC3) and rescuing the damsel in distress that is the main characters close friend/love interest (DMC4).
Oh please its not like DmC ripped off anime either.
Regardless what you say...The.Same.Sh*t.
They ripped off They Live with the hidden messages that pop ups showing the real intent behind billboards and various such phrases as "We have awoken" or "Stay asleep" (stuff that are trademarked and iconic to They Live) and Tameem himself admitted drawing influence from it and other movies.
Also DMC1 and DMC2 original.....ahahaha!!! Actually you might be right on that one although the concept for an evil business man out to take over the world or become immortal, a once sealed sealed demon king planning to invade into the human world and conquer it, and an avenge my family plot has been done many times before but I can't pinpoint the exact source where they could've drawn influence from (although Kamiya admitted taking influence from the Cobra manga and Blade comics). DMC2 seem to be the most original as if I don't think Itsuno got any direct ideas for any characters, plot point, or story elements directly from any known piece of media maybe literature such as the Bible (Argosax representing Legion) and Norse mythology (Bolverk and the wolves).