Forza Horizon: Once I got 100% I returned to Forza Motorsport 4 and raced in B-Class Hoppers and Custom Public Lobbies for 2 years.
Forza Horizon 2: I got my console about a month before the release of Forza Motorsport 6 and moved over once that game was released.
Forza Horizon 3: I show up every so often for Forzathon, but other than that I’ve been playing more games in general. Right now I’m playing a lot of Street Fighter V and Final Fantasy XV on PlayStation 4.
What usually keeps me going after 100% completion is a Multiplayer environment that is consistently challenging and encourages me to develop my skills. With Forza Motorsport 4 that means racing in Public Hoppers with the intention of winning most races. Despite the usual issues with crashers the Custom Public Lobbies feature meant that I had enough enjoyable clean racing to keep me going for 3 years. I had a similar feeling in Forza Motorsport 6 but the absence of Custom Public Lobbies meant that I had to deal with a lot more incidents and immature racers in the Hoppers, and as such I stopped playing about 6 or 7 months into that title’s release. The only game that has given me that “I need to come back to this” feeling (after 100% completion) in the past year was Rocket League.
In short: If the Multiplayer is “good”, I’ll keep playing. Forza Horizon 3’s Multiplayer doesn’t feel fun to me, as it got repetitive very quickly.