Third, if it's possible, reward people who played the first game. There will always be some percentage of players who are going to play your sequel without first having played the original. The majority, however, will be players who've been with you from the beginning. The Ratchet and Clank series lets the player bring over save data from the previous game, and this can make a difference in gameplay. The Penny Arcade games do this as well, and while it may seem minor, little things like this can serve as a small "thank you" to your player base and make them feel good about keeping interest in the franchise.

Finally, revisit old design docs from the previous game. There will always be cast-off ideas that didn't feel right for inclusion in the first game; maybe with a fresh outlook they can be dusted off and reworked into something new. Sometimes the process of merely looking through old concepts can spark new ones and help push the franchise forward into novel territory.

Sometimes it's the lure of furthering the storyline that creates a sequel. If there were any unanswered questions or unresolved plot points from the first game, a sequel can help fix those loose ends. Again, there are good practices in effect that will help make a continuation of the story feel natural.

First, make sure that your plot follows the previously established internal logic. Nothing destroys a fictional world's reality faster than having the characters act in ways that seem antithetical to the way they acted in the first game. Or when an event occurs that feels like it was arbitrarily dropped in just to see what would happen. Unless your franchise is episodic -- each game's story is self-contained -- then it's important to keep events believable within the framework that you created in the first game. But even an episodic franchise should definitely keep a consistent thread flowing through the game's story, some way to tie the series together other than by the simple expedient of having the same characters in the game.

Second, dive deeper into the lore of the game. History provides a rich field to be farmed, and it can be your friend when trying to flesh out your world. Sometimes you'll find tiny moments that can turn into major plot points with the proper massaging. And this also serves to broaden the game world and make it feel more real, which always helps with immersion and keeping audience interest.

Finally, resist "character creep". This is when new characters keep getting added to a game franchise in an effort to keep it relevant. The Sonic series has suffered from this; remember when it used to just be about Sonic and maybe Tails? Now the cast of characters is so large it's bloated and unwieldy. The focus needs to remain on the player character. Bringing in new antagonists all the time just dilutes the power of the hero; remember the original Batman movie franchise before Christopher Nolan revitalized it? Villain after villain, at least two per movie, there may have been three at once for one of them, I don't recall. If the bad guy is more interesting than the hero, it's time to rethink the series.

If you can manage to keep track of all these things, you stand a good chance of keeping the audience not only satisfied with your latest sequel, but provisionally looking forward to the next one as well, and that's a terrific place to be.