I've been thinking about it, and I know how I would've ended this series. Not going into too much detail, but it would've went like this:
- The story would focus on the redemption of Gabriel. Some of the ideas I'd keep would be Zobek's goal to usurp Satan as well as Alucard making a deal with Gabriel.
- Main plot would be Gabula seeking out a weapon(ultimate holy weapon) composed of the holiest of relics on Earth(one relic would be the Spear of Destiny, another the Combat Cross, among other fabled artifacts), which can forever silence Satan(as well as Gabriel himself). Zobek tells Gabula of it and how both the Brotherhood of Light and Satan's army seek it.
- Brotherhood of Light would have a bigger part in the modern day. Alucard would act as a mediary between Gabula and the Brotherhood, who'd, for a brief time, ally with each other only because of their goals are the same(seeking out the "holy weapon").
- Victor would still be part of the Brotherhood and have a greater role in the overall game.
- About half-way into the game, the Brotherhood gets slaughtered when Satan's army attacks their headquarters. It's a massive battle where Gabula, Alucard and Victor fight side-by-side. Victor dies heroically. End of the Brotherhood.
- Holy weapon is discovered, but Zobek steals it and wounds Gabula with it. Turns out it's incomplete(missing one part, so it doesn't kill Gabula). Brief portion where Alucard offers Gabula some of his blood(like Gabula did for Trevor in MoF), then Alucard goes to find the missing piece(control Alucard for this section).
- No Leviathan. Satan is summoned back to the world and creates his own version of Dracula's Castle, Pandemonium, which acts as the final dungeon. Full-scale apocalyptic scene happens(the world feels this inbalance and natural disasters occur in various parts of the world). Gabula, at full strength again, is joined by Alucard who helps storm the castle. Alucard keeps a lot of Satan's army at bay while Gabula travels deep within the heart of the twisted keep.
- He confronts Zobek again, Death battle ensues and Gabula wins, taking back the holy weapon. Forms the full version of the holy weapon with the piece Alucard gave him.
- Final Battle agains Satan. First in human form, then in demon form, and last, Dragon(Gabula) versus the Beast(Satan). Satan's Beast form is based on the multi-headed dragon designs in the art(as well as the infamous apocalyptic imagery of Revelations). Totally epic battle.
- The end has Gabula and Alucard(together) impaling Satan(in mid-air) with the weapon, causing a hole(The Pit) to open in the space below as they fall. It seeks to engulf them(a realm of pure nothingness where no evil can escape from). Satan fights it though, so Gabula and Alucard choose to Sacrifice themselves by cutting their palms on the weapon and binding their blood to it, in turn, binding Satan with their blood and keeping him down. What they don't expect is, not being beings of PURE evil, by binding their dark blood to the weapon, it's actually their dark side that is bound, literally pulling it from their bodies and forever sealing Satan within The Pit as Gabriel(not Gabula anymore) and Trevor(not Alucard... hehe) escape as humans.
- Basically, the prophecy of Revelations was altered, and now the world is able to move forward into a new direction. Gabriel and Trevor are able to live out their lives in peace.
Though, for me, the whole last part I'm on the fence with. Either Gabriel and Trevor live as humans, or when they are shed of their dark side, they both ascend into the heavens(greeted by Marie).