What Makes a Good Sequel?

Vivien and I regularly create characters we can't stand to leave behind. In fact, we have eight full-length sequels (and one novella length prequel) to Master of Obsidian because we couldn't walk away from Jesse and Gideon. We have finally decided how to end the series after a total of ten books, and it pains me to know that it's coming to an end. I'm going to miss Gideon and Jesse and Emma and all our secondary characters terribly.

In a few weeks, I'll be flying to California so Vivien and I can work on the 2nd sequel to Chasing Silver. We're also currently discussing a sequel to our Samhain title Trinity Broken. We're currently finishing up a full-length gay historical called A Hidden Beauty and already brainstorming sequel possibilities. As we discussed the possibilities, we were led to one overwhelming question: Why does the romance have to be about getting together? Why can't it be about staying together?

I think we strive to make the conflict between our characters organic and inevitable given who they are. There's going to be problems when you're a human and you fall in love with a vampire--those problems will be amplified in a BDSM relationship. There are going to be issues in a menage relationship when 2 of the people are shapeshifters, and one is a "regular" human. There are going to be challenges when you fall in love with a woman is from 75 years in the future. Etc etc.

But it seems to me that most sequels do not feature the same characters from book to book, as though the "Happily Ever After" ending was 100% genuine, and two people will never have another conflict again. They tend to feature secondary characters that were introduced in a previous book, or simple rehearsals of the same scenario over and over. I can see the attraction of reading those, but generally I write sequels because I miss my characters.

Sometimes I wonder if we're potentially sabotaging ourselves. Perhaps writing sequels about established characters presupposes that the readers love them enough to visit them again (and again and again and again in some cases). Since we don't write with a plan in mind, we also run the risk of something horrible happening. Clearly, it won't do to write a book about people falling out of love!

What sort of sequel do you prefer reading? Writing? And further, why can't a romance be about staying in love?