Thinking about it, I've always been interested in the history behind "Tony Redgrave". Didn't Kamiya say that he was a journalist that Dante respected and so used his name as an alias? He could be investigating the demons and comes across a younger Dante.
Or alternatively you could go the route of the non canon novel where an amnesiac Tony Redgrave (Dante) is discovering his heritage via the antagonistic Gilver. In fact an adaptation of that novel would probably be the safest bet to draw people in.
There is a bunch of interesting story ideas, concept art, costumes etc in the artbooks that might be cool to reuse plus its a good PR tactic to get disgruntled/skeptical fans on your side.
On your idea itself, using the novel would be cool but it presents a different look at Dante/Vergil and their past that what came after DMC1 so i think you might have to shift things around in later stories to make it feel right. Using the novel as what happened a year before DMC3 would shift the characters and their struggles a lot.
Maybe have Dante pretend to be Tony's son Lucas
and use Tony's contacts in journalism to hunt demons might be a good way to go and a shout out to Bayonetta. Heck you could even have Tony be a relative of Eva who takes him in after she dies.
In the artbook, there was gonna be a different lead for DMC2 with a green jacket and the anime had a demon-human couple so maybe focusing on that potential character could be spinoff material.
Or just start with Nero and make him the audience surrogate like my dmc4 anime pitch.