OK, so The Adventure Rebirth included many elements of its source material--even including some enemies/music from it's Belmont's Revenge GB sequel--but it also removed a lot. Consider a Belmont's Revenge Rebirth that takes all/most of the level/enemy/music elements left unused and remixed into a new game. (And, imagine if said game upped the ante by including SCV-IV platform-swinging and some Gameboy rope platforming every now and then. Finally, just to round things off, let's include a few of the better touches from Castlevania: Legends and a few surprises true to the "Rebirth" idea). Keep in mind that a password/save feature should be implemented in this case.
In my imagination, here's one such outcome (with general reasons behind choices):
LEVEL 1: Forest to Mountain Graveyard (based on The Adventure Level 1)
Midboss: Gobanz (Classic GB1 boss).
Boss: Puwexil (its saliva is what is creating the sky-dripping "madmen," and it's a rarely-used skull boss that fits in a graveyard).
Music: Battle of the Holy
LEVEL 2: Caves to Rock Castle (inside the mountain from Level 1).
Midboss: Mummies (because Rock Castle has Egyptian motif)
Boss: Iron Doll (Classic GB2 boss)
Music: Darkness, Psycho Warrior
LEVEL 3: Crystal Castle
Midboss: Orphic Vipers (commanding water creatures)
Boss: Darkside (Classic GB2 boss)
Music: Okada 2 (Bloody Tears Remix) from GB3 CV: Legends (It has a pace that is like New Messiah, already used in Rebirth 1, and is actually very different than most Bloody Tears arrangements).
LEVEL 4: Plant Castle (waterfalls of previous lead to lush garden below)
Midboss: Medusa (snakes in a garden; it makes sense)
Boss: Kumulo and Nimbler (Classic GB2 Boss)
Music: Ripe Seeds
LEVEL 5: Cloud Castle (gardens lead to sky like Jack and the Beanstalk)
Midboss: Death Bat (Classic GB boss that flys)
Boss: The Angel Mummy (Classic GB2 boss)
Music: Praying Hands
LEVEL 6: Castlevania Entrance (mix of GB1 Level 4 and GB2 Level 5)
Midboss: Cerberus (ala Dracula X SNES; guard dog to entrance)
Boss: Bone Dragon King (The Classic GB2 boss that comes from above and below while screen scrolls)
Music: Original Sin
LEVEL 7: Chapel (GB2 Level 6)
Midboss: Death Dragon (Classic GB3 boss)
Boss: Soleiyu
Music: Revenge
LEVEL 8: Death Tower to Final Leg/Keep (mix GB1 Level 3 with GB2 Level 7)
Midboss: Death
Boss: Dracula (GB2 version), Giant Bat (GB1 version), Dracula's Ghost (GB3 Legends version)
Music: Death Fair, Road to the Enemy #2
So, thoughts? Or your version?