Ideas
Here are some ideas in sales pitch form
minigames:From unlockable ones to need to do ones or even buyable ones, minigames galore.
multiplayer:Either kill your partner or kill with your partner.
pets:cant get a partner? well then buy a partner.
more weapons:just making your weapon more powerful not doing it? well then get a diffrent weapon with more attacks.
Thats it. Well I hope you at least like one of them.
And the game is still great