Writing a sequel to my first book now. I've never written a series before, but it's presenting wonderful challenges as well as opportunity.
You have to show how the characters have evolved from the first book. They were important, so you can't just leave them in the cold. Readers deserve to know how those characters are doing.
You have to introduce new characters. Obviously, readers can't expect to get to know a few dozen characters well in one book, but to introduce new characters and make readers love them just as much as they did original ones is tantamount to starting from scratch again.
Fleshing out broad webs of connectivity. Relationships are necessary with an ever-expanding world full of (hopefully) interesting, relatable, and lovable characters. These relationships have to be handled with kid gloves if they're going to feel genuine.
But with these kinds of challenges, the opportunity to spend more time in your world presents itself. You have more time to play, to explore, to imagine, to walk the streets and live these characters' lives.
It's like coming home or wearing an old, worn flannel shirt. There are times I don't want to stop writing. I've never felt that. I think I understand why writers come back to their worlds for strings of books.