If you want storytelling , player should have reason for stay in the game. Also slow actions are not good for it. Voiceover should not reduce the player's patience. You can try make the game fast as much as possible.
Every "block" has special features. For example "R"s change the gravity. You must hit them(like in Super Mario). But "C"s and "O"s are more complex. With "C", you can give cooldown to blocks; "O" is for choosing the block whichone you want.
"Close" kills gain more score and that actually great factor in games like that. Can make game background color different to zombies' color but health and time bar fixed that. Any music and SFX(charge absolutely) can make better the game. A game that has reflex based mechanic and reflect the theme.
A 2D platformer game that has attractive aesthetics and smooth gameplay. Any music and SFX can make better the game. Also if planned difficulty is not decreases of mana, obstacles can make more hard.