Make it boring and monotonous would be my guess.
Just like FM3 and FM4 career modes were. Turn 10 is as lame as making Career mode as PD is nowadays.
Having said that, I must say that Forza Horizon is probably the most entertaining racing game I have played since ... probably GT4 and last gen NFS like Underground 2 and Most Wanted.
It really shows what kind of experience can be developed out of Forza Motorsports assets, the game is consistent, addictive, varied and well paced, providing different kinds of events and game modes that don't let the game stagnate into the grind that main series get at most times.
There is a sense of flow in the game, it's packed with a lot of events, characteristics, settings, items and other game elements that helps the game stay fresh at all times, from racing in sim-like conditions to free cruising and earning points to gain credits, Horizon really does nail the whole concept of "fun", something that Forza games have failed to achieve since FM2. Part of this could be attributed to the new layout that has been given to the game, and the new characteristics that has been brought by Playground games, which seems to be a pretty strong to keep going with this Franchise.
The game shows a lot of elements from Project Gotham Racing series(most noticeably the stunt points that are pretty similar to the Kudos system from old PGR games, how the events are structured and how the physics seems to act sometimes), off road racing in this game is also pretty well modeled(not sure if old CMR developers had some involvement on this project, but the rally racing is really well portrayed here, not sim-like, but portrayed decently enough to provide a varied experience in all kinds of terrains).
Cars maintain some of the driving characteristics, and the game itself keeps a physics model capable of portraying these characteristics, while making the game a bit arcade, but approachable to both sim and arcade sides, the events set for it are great and the setting is also great(comparisons with TDU has been done, Horizon's map is packed with a lot of great driving roads and diverse scenarios, while in TDU this was limited to a handful of roads that were in remote places, in a very repetitive and sometimes dull scenario).
This game is great, makes me look forward to next project involving Playground Games, and a future iteration of the series. They seem to grasp the concept of "game", something that Turn 10 have failed to address for the last 5 or so years. Money grabbing tactics like 1st day DLC and some significant omissions in the car list(like Jaguar XJ220) exists in the game, however the car list and the game itself manages to keep all these little issues aside and overlooked(not failing for those, car list is pretty strong as it is, and seasonal passes are a robbery anyway).
Other little things like tamed down engine sounds, and the Soundtrack list are indeed rather weak points in Horizon, however custom soundtracks seems to patch this issue nicely, and cars still have good sound(even if they are really lowered down).
In conclusion, this shows how a proper developer can make a fun game, and how a fun game should be, in my opinion this should be a must game if you own an Xbox, I personally think this the best game to come out this year and makes me expect for a new game in the series. This was "Colorado 2012", I would love to see an extended map in something like "Swiss Alps 2014", it would be a great to see something like that in the future.
Almost forgot, I think the game has a decent length, I don't fast travel and I do earn a lot of credits for sponsors challenges(not to mention that free roaming allows me to test car handling and such), I keep the game in a considerable difficulty and I honestly don't get why people is beating the wrist levels so fast, superficially there are the wrist-bands, but there is much, much more in horizon, something that gives it an incredible re playability.