I'll add to this post over time.
About the monthly rewards: +1 on the sacrificial heart idea, how about, since there are 6 precious soul characters, we spread them throughout the month, (1* Gold Idol on the 5th, 1* Heart on the 10th, 3* Gold Idol on the 15th and so on)
About the store ideas: PLEASE give us a gold boost similar to the XP boost
An option to change sound settings in battle would be nice as a minor change. Also, maybe an option to use randomized game tracks when you're playing a level if you want to hear something different.
Edit: actually since the calendar has 24 daily rewards in one month, why not fill out the last 6 days with the hearts and idols?