With Thrillville, we were lucky to have so many gameplay types that naturally fit within the theme-park fantasy already. You can imagine yourself playing bumper cars, racing go-karts, building roller coasters, selling ice cream, etc. So it never felt like either a design stretch or a strain on authenticity to create game with all of these elements.
Above: This screen is from the PS2 version of the game
Because our team has worked together on simulation games in the past, we've had a head start on thinking about Thrillville as a game greater than the sum of its parts. We knew that there would be three basic things that drive the game world: happiness, maintenance and money. So, each gameplay mechanic serves at least one of these game needs, and some of the more rewarding ones serve more than that! Knowing this, we had a place to work from on the level of basic motivation.
What has been trickier has been developing a hardy "ecosystem" for Thrillville. Each of the core elements of the game, whether it's building, talking to people, playing games, managing staff or minding the finance, needs to both feed off the others and give something back to you as the player. This system of interdependence isn't something that can be shoehorned in. You need to commit to it early, so that everyone on the team is designing and implementing with it in mind.