Castlevania II - Belmont's Revenge (GB): Hard Mode
It's a great game, but the last couple of bosses aside, it lacks the challenge of its predecessor or most other Classicvanias.
The game's main problem is that the player easily becomes overpowered with the fireball-whip and subweapons on top of that.
One thing I'd like to see would be a standard whip-only mode (no chain- or fireball-upgrades). That way the sub-weapons would make more sense. I also think there should be less hearts overall (half the amount, or less).
Some enemies, like the big knight-types and the fireballs spat out by those reptile heads should be way faster (at least 1,5 times). The first 4 regular bosses could use some more hitpoints, be faster or have some added regular enemy types (e.g. bats) appear, if the latter won't slow down the game too much.
Another mode that would be cool would be "The Adventure"-mode, with rules closer to the first GB game: hearts for subweapons should be very rare, all enemy hits should downgrade the whip and take away hearts, too (like the floating jellyfish do), spikes should back to one-hit kills. The above mentioned enemy tweaks should apply, too.