So, all the races in fh4 are one time things except for when there are seasonal championships and rivals. Additionally, after completion getting new cars and money becomes a constant grind up and down a god damn boring highway. A cool way to reuse the content and subvert the grind is to make and optional Season 2 version of all the races (which would be activated by choice somewhere like the festival). You would also be able to activate road, cross country, dirt and street season 2 separately.
These races would be on any difficulty option without rewind and without any replay option, so you’d playing all road races from the begging like an arcade. You lose and you are out. Before you start you chose what class the cars should be in. All Driver-tars will be tuned to the top of that class (the higher the class the better rewards). The driver-tars also gotta get fixed (so they don’t get extra power output/traction when you go on higher difficulties). There could be really expensive AS cars, money and super wheel spins locked behind progressing in the game mode (but no exclusive cars since it is optional). The rewards should give a lot more than just the normal money for winning since you only get one chance to progress. And the multipliers should be exponential for each increase in difficulty due to the circumstances. You get all the rewards after either from being knocked out or winning the last race (or deciding to quit).
There could be many achievements locked behind season 2 and it would provide and endgame for players who dislike the online system. This would not be so hard to implement (i would assume) as it is almost no new content. And like i said, it would subvert the grind from just doing highway races in a compelling way.