Before becoming a mystery/suspense author, I was and still am an avid mystery/suspense reader. Perhaps I'm just a detective at heart. I love following the subtle clues, fleshing out the characters underlying personalities and guessing--and then waiting until that last page and finding out if I was actually a good detective, or because of my bungles the killer would have gone free.
A good mystery needs a good plot, and possibly several sub-plots to keep the reader guessing. So if you're starting a mystery today, consider the following:
1) A believable protagonist. Every believable protagonist makes mistakes. They don't walk from point a to point b and capture the killer. They struggle, both with the clues themselves as well as their own inner struggles which make you like them or love them and want to follow them all the way to the end.
2) A really intelligent bad guy. Criminals are not stupid. In fact, many are brilliant. Your bad guy must be able to lead your main character on a chase--which of course should have a good ending, the killer punished by being caught or well. . . .
3) Clues. Readers are not stupid either. And we love to follow clues. Figure out the answers and use our own imagination. So leave plenty of clues to allow your reader to make an educated guess, and follow you to the end--just to see if they're right.
4) Reality in place and time. Research your period. Not just the architecture, but the language, the dress. Mini skirts weren't prevalent in the 1920's. At least not on the street. A mistake in time and place can stop a reader dead cold and make them wonder what you were thinking, or what the heck did they just read.
Should a mystery have a sequel? Sometimes yes, sometimes no. It truly depends on your character and your readers. After writing The Gifts, many of my readers wanted to know more. More about Jacody. What caused the split between him and Gavin. Loved or hated, they wanted more. So I didn't write a sequel. I wrote a prequil. The story behind the story. I sort of did a Star Wars. Wasn't my original intention, but I wanted to give my readers what they wanted.
Sacred Secrets was a character driven novel all the way for me. To be honest--I guessed, but I truly didn't know 100% for sure who was the bad guy--until the last three chapters. Was I surprised? Yeah, I sat back in my chair and looked at Coby and said, "Geeze, I would never have guessed that."