The formula hasn't much changed for the Forza Horizon series since it debuted in 2012. Introduced more as an arcade-sim hybrid focused on fun factor and social elements than its big brother, the simulation-minded Forza Motorsport, the two have alternated years with new releases in the time that's followed.
The formulas behind the two haven't changed for a reason: They're basically perfect.
Forza Horizon 3 builds off its two predecessors while remaining very familiar. Everything great about Horizon has returned, but with some new compelling features built on and with the racing taking place in the best setting yet.