A few ideas: Differing endings depending on your streak. A little animation could play when you hit a win streak of maybe 3 or five when you win you'll turn into a vampire might be neat.
I've found that more often than I use the + abilities for finding the vampire, I use them to try and safely nab the square or < abilities. Maybe they could affect an entire row in later stages.
Seeing as how you've added a new win streaks mechanic, the game could have a difficulty progression based on your streak, adding later vampires based on the streak.
I've said this before, but if you're considering a sequel a mobile port or steam release wouldn't go unnoticed.
You could add sprites for Ace, 10, 11, 12, Jack, King, Queen, and have a 4x4 mode, with vampire at highest value. Might make the final vampire more difficult and intense, tho you'd have to make the square a 3x3 for that or else it would be unbalanced.