There are various ways to sell sequels to games. I’ve sold 2 sequels myself (Democracy 2 and Kudos 2). The approach I took was this:
- Take what’s good about the original game, and expand upon it.
- Fix any of the design or technical limitations of the original that were too big to just patch.
- Add some new features, and polish the stuff that is already there. Maybe with a bigger budget this time.
- Respond to tons of real paying-customer feedback to make changes and improvements to the user experience.
In short, make a better version of the game.
Or of course, you can be cynical as an ambulance-chasing lawyer, and just turn off the servers for last years version, and then sell the same game again with some extra bump maps and a new logo.
Seriously, running game servers is cheap. How do people think all those FreeToPlay games cope? This is so, so cynical.