Couple of suggestions that would make me invest more in the game....
I am fortunate enough to have my name listed in the screen on the inside of the start gate, and its quite a buzz.viewtopic.php?f=16&t=1688&hilit=name+in+game
You could maybe ask peeps to donate and get their name in the game, on a big screen by the track side or just left as is.
This could be a small charge for a lottery type thing, where names are entered into a draw or a big charge type thing where you definitely get a mention in game.
It may be extra work to implement but I think some would pay to get their name in the final game.
Second Idea -
I missed out on the very early backer status and therefore can't access the steam beta versions without a key ( I know we had access to beta MP)
Is there a way you could reset that key, and then only give the code to the people that donated extra if they arent already involved in the very early access beta.