Its impossible really.
We really don't know what became of Vergil. Dead. Inside Nero. Wandering somewhere.
And Dante was pretty much there unlike Frank West who was pretty absent from DR2.
The best is make a Special Edition where Dante has his own campaign separate from Nero and focus on what Dante was doing prior to being playable.
It will have to feature elements from the DMC4 manga which is considered by Bingo Morihashi (the writer for DMC4) the complete version or his original script before the cuts.
Throw in a new mode or something, make Trish a playable character like Vergil in DMC3 or have like 3 missions in Dante's campaign and useable in BP or at least make her playable in Dante's stages. Throw in new boss fights for Dante like a Nero boss like in the Iphone game and like 2 new bosses. New stages and areas for Dante to explore or at least did what the Iphone game did and create new environments for in that game Dante took a different path than Nero. As well as 3 new enemy classes.
Dante has 15 missions (by adding 8 to his missions) and Nero has 15 missions (2 more missions) making it 30 missions.
For Dante
Mission 1: The story starts and he fights Nero as a boss fight. Fights end and Dante head out to the HQ to finish the job while thinking about Nero.
*Nero boss fight*
Mission 2: He ventures around Fortuna City while taking a different route and being able to skip Berial and the Hell Gate and ending up near the Castle. On the way he find his trusty Shotgun that he left lying about.He goes to the Castle but finds a different way to get out and goes to library to do research on the Order and learn why his brother is connected to this place too but is attacked by an Alto Angelo.
*Scarecrow Arm and Scarecrow Leg are introduced*
*Player gets Coyote A*
*Frosts appear in the middle*
*Alto Angelo are introduced*
Mission 3: Dante attempts to leave the castle and make it for Order of the Sword HQ but feels a shock wave and accidentally DTs well due to Nero activating DT at the same time when he is facing Agnus. This causes a Faust to appear and Dante must face it. Dante makes it out of the Castle and arrives at the forest in a different route Nero took.
*Mephistos are introduced at the near beginning*
*Faust appears at the near end*
*Player Unlocks Devil Trigger for Dante*
Mission 4: Dante meets Nero again and goes into the forest. There he takes an altered path as the main path was blocked by the trees but he ends up at Fortuna HQ and encounters an Bianco Angelo with Alto Angelos who attempts to stall him while the bridge gets ensnared by the vines. He defeats them and advances and passes the bridge right before the vines tries to ensnared them. Then Dante makes it up to the Ascension Hall he is attacked by other Order generals he slayed before who turned themselves into demons revealing their demonic forms and are the next enemy class. So Dante basically fought the remaining Order soldiers and killed them all. At the end Dante faces a head Order general, a new character with a name that matches the same Catholic terminology theme Credo, Sanctus, and Agnus followed, who was going through his ascension ceremony and transformed into his Angelo form making him a new boss. Dante defeats him and asked him to reveal rather important info in which he reveals that Nero has awaken/revived Yamato and the Order planned to use him instead to complete their goals (aroun this is the same time the Nero VS Credo fight already happened and he was facing Agnus by now). Dante curious wanted to know more but the general soon blew himself up to deal with Dante before saying even if Dante wiped out their entire forces the plan will soon be complete.
*Bianco Angelo is introduced at the beginning*
*New enemy class White Knight is introduced*
*New Boss Fight: Random Order General*
Mission 5: Dante waits in the ascension room for Nero and they fight and you know the rest.
*Nero boss fight again but this time with Devil Trigger*
*Trish becomes playable*
Mission 6: Trish tells Dante some valuable information on the Order (almost everything really) and decide to go help Nero while clearing the enemies and playing as Trish while Dante lets her clear the way. You know the rest.
*Trish is playable with Luce & Ombra, Martial Arts like in DMC2, and a Devil Arm she stole from them*
Mission 7: Dante exists the Order base.
*Mega Scarecrows are introduced*
*By this time the player should've gotten used to Dante controls on a basic level*
Mission 8: Dante kills Echinda and gets Gilgamesh.
*Assasults are introduced at the near beginning*
*Chimera Seeds are introduced*
*Echinda boss fight*
*Player gets Gilgamesh*
Mission 9: Dante leaves the forest and ends up fighting a Blitz.
*Blitz is introduced*
Mission 10: Dante faces Dagon and gets Pandora.
*Dagon boss fight*
*Player gets Pandora*
Mission 11: Dante makes it out of the factory and head to Berial.
*Those cut fish things appears*
*Basilisks appears*
*Berial boss fight*
*Player gets Lucifer*
Mission 12: Dante heads to Agnus and beats him.
*Agnus Boss Fight*
Mission 13: Dane ventures deep in the underground area which is brand spanking new area and makes it to Yamato but is stopped by a new demon class created by Agnus to protect Yamato as well as other demons. Dante defeats them and gets Yamato.
*New enemy class artificial demon made by Agnus*
*Player gets Yamato*
Mission 14: Dante heads o fight the Savior while Trish protects the civilians. Players play as Trish in the longest mission in Dante's campaign as she fights an horde of demons to defend and protect the citizens of Fortuna. He heads from the church to the port where the citizens being held or secured in the factory and shaft after and legions of demons are heading there and its like a gauntlet or stream of practically almost every enemy and she fights them all and after wiping them all out but Lady and her boat arrives (taken from that scene in the DMC4 Pachislot machine) to help Trish out and Trish heads out to see if any other civilians were missed and reaches an area and fights more enemies and encounters a new boss or a powerful demon that escaped from the True Hell Gate. A really powerful demon and the strongest of the demons that made it out. A very powerful demon that surpasses even the likes of Berial, Echinda, and Bael/Dagon and was a huge threat that even Sparda took it seriously. Trish defeats him claiming he was all talk.
*New enemy class*
*New Boss*
Mission 15: Last Mission. Dante faces the Savior in 3 phrases now. First phase is destroying the crystals on his body, 2nd phase the actual fight, and 3rd phase is another boss fight with the Savior but with a more fast beat and heavy orchestral music or a new track. Dante gets Yamato inside the Savior and decide to finish stuff up where he is and is later flung to the ground and the Savior descended to the ground and Dante engages it in more intense fight giving it his best and praising Sanctus for giving him a worthy challenge he seem to have never had in a long while. Nero defeats Sanctus and escapes and crushes the Savior and they all head home the end and the secret ending is Dante's basic ending while Nero get's a new secret ending.
Now Nero
Mission 1 - Mission 11: Same as before
Mission 12: Nero is lodged in deep sleep and reminiscence on his past in his head reflecting on when Kyrie was attacked and reliving moments in his life while facing various demons in them. Transversing a different Mitis Forest and Fortuna City in his own head.
Mission 13: Nero meets the spirit inside him AKA Nelo Angelo/Vergil and faces him and after beating him Nelo Angelo asked why he wanted power. Nero answered to protect Kyrie and he'll do anything even become a demon to get stronger. The demon liked his answer and told him to claim his power and soon Yamato appeared and Nero heard Dante's voice and Nero soon woke up seeing Yamato took it and used it set himself free.
*Nelo Angelo boss fight*
Mission 14: Same as Mission 19.
Mission 15: The Diaboliticus Sanctus boss fight and then the last part in Mission 20 where Nero crushes the Savior face is now a full fledged boss fight where a new track plays as Nero defeats the Savior and Sanctus once and for all. Beat game and cue end credits and the secret ending this time shows what becomes of Fortuna and Nero/Kyrie after the events of the game told or narrated by Kyrie and its basically what the DMC4 manga said (Kyrie opens up an orphanage to help the kids who lost their parents and homes during the incident, Nero is open with his demonic heritage but is seen as a monster or the one responsible for the whole mess by the general public even if Kyrie tries to reason with them about it, and Nero opens up his own shop with a neon sign from Dante titled "Devil May Cry").
That is it the end.
