You are probably getting annoyed with all my ideas but this one is short...compared to 1 and 2!
1.Multiplayer so just add a few servers and make sure you can create private servers so you can play with friends.
2.refueling (I'm thinking of a slider 10-20-30-40-50-60-70-80-90-100-90-80-70-60-50-40-40-30-20-10 and this is in percent so you could decide the amount of fuel in each vehicle).
3.A career mode would be cool but keep the sandbox and what if you did a space race with a budget and a friend.
1.Multiplayer so just add a few servers and make sure you can create private servers so you can play with friends.
2.refueling (I'm thinking of a slider 10-20-30-40-50-60-70-80-90-100-90-80-70-60-50-40-40-30-20-10 and this is in percent so you could decide the amount of fuel in each vehicle).
3.A career mode would be cool but keep the sandbox and what if you did a space race with a budget and a friend.