Thank you for offering the topic of customization. I would forward the following ideas for consideration.
Allow for more customization of machines once clients reach the top level. I've beaten all the tracks and gotten all the extras and I haven't been able to make my bike any better. Part of racing games that makes them enjoyable is modding out the machines. When I raced actively, this was part of the experience. So being able to modify the top end bikes with new suspension, exhaust, airflow, tire selection, weight savings, fuel mixture, engine low end and top end, gearing and torque would make the game more complicated and challenging.
Allow for more variable track conditions and soil types. The track conditions and soil types seem the same across all the levels and tracks. Imagine if there were sand sections, hardpack and mud as well as the general track conditions and soil. This also creates an opportunity for some intense racing as track conditions may not just favor speed, but also technique and bike set up(if we're able to customize the bikes).
Allow for some really high speed motard type tracks the combine asphalt, concrete and dirt. There could also be hill climbing, trials or downhill courses that would be exciting. We could customize the bikes for the different types of challenges.
Thank you for your time and consideration.