more variety in challenges
Safety car comes out, you need to keep your tyre temp up.
More variations in chicanes and hairpins, maybe based on the track raced on
DRS - you need to open your drs as you cross the line to complete the overtake, maybe add a corner before hand to make it more challenging.
Pitstop - press the brake at the right time to stop on your marks
If you're in the bottom 6. The opposite to the backmarker challenge where you have to let the blue flag cars past by pressing the brake, but not the other cars you're racing
Its just a few suggestions, no doubt you've thought of others already. Its a great game, early days of course, but really enjoyable
Some great ideas - esp the reverse back-marker challenge.