My perfect ending had a lot to do with Dracula getting all his power back and kicking Satan's fully powered form as was hinted in the epilogue of the first game. I had other ideas running about until I played the entire game, where I threw some out and wished I could rearrange certain portions to make more sense. Basically, I wanted the Inner Dracula fight to happen closer to Zobek just so there's some explanation as to how Dracula suddenly jumps from weak to strong enough to take down the lord of the dead in hours. My justification for it would have been instead of letting Inner Dracula escape, Dracula actually absorbs the blood into himself and gets a huge boost to his powers, symbolizing how he has conquered his dark side and was now at peace with both sides of his being. This in turn makes him both symbolically and literally a more complete being than when he first began his journey. By this point of the game, the players should have had access to a fully controllable Dragon Form, which would have been glorious to use on Zobek/Satan.
With the Inner Dracula segment concluding the significance of that subplot, there should have been a change in the castle's minions so that they're no longer possessed and thus support Dracula's cause again. Fast forward to the Satan fight, I had no problem with Satan possessing Alucard as long as there was an actual final form after that. Satan using Light/Shadow magic like he did in the first game would have been an excellent throwback to the finales of the previous two games and would have added just that extra bit of consistency to the end of the trilogy. After Alucard is defeated, there should have been a cutscene afterwards acknowledging that Dracula had indeed returned to full power, like say the castle demolishing the city and restoring it to its former glory. Even a little scene with the castle monsters rushing out to fight the hordes of Satan would have been a neat cinematic, showing that Dracula, his forces, and Victor (who should never have died and should have had some modern BoL backup) were literally beating Satan and his army back to hell. This would have been a better justification for Gabriel turning into Dracula--so that he and the remnants of his army could fight back a greater evil into submission. In other words, an actual display of chaos in the final battle for the earth instead of a depopulated city with some fire and debris and barely enough demons to call an army.
So now with this going on, and Satan understandably pissed, he transforms into a monstrous second form, like the 7-headed beast in his pages in the art book while Dracula transforms into a permanent (for this segment of the boss fight) Dragon form. The two duke it out and while that's happening in the sky, Alucard wakes up in the ground and sees the carnage between the two armies. He then helps Victor while the two big bads fight it out. By the end of the fight, most of the armies should have been eradicated on both sides, with Victor and Alucard showing signs of fatigue. Dracula and Satan, having unleashed all their power, are now too weak to maintain their forms and have to resort to cqc. Unable to use even his Shadow Whip, Dracula then picks up the Combat Cross and kills Satan with it, ending with the stab that destroys the immediate vicinity (if Raisa exploded with her explosion, I don't understand why the other acolytes didn't). The castle collapses, as Dracula, who has been dealt some heavy blows himself dies but not before giving Alucard his "last" sentiments. Alucard and Victor search for the remains and see nothing but the Combat Cross and so vow to finally end Dracula's suffering once he inevitably comes back.