1: At what point do you think a sim (or semi-sim) racing game is attractive to you?
When it offers diversity, e.g. different car models need different styles of driving, feeling of actually driving a car, instead of having several different sizes of bricks put on 4 wheels. Basically meaning every car is different from each other and they actually have some different charasteristics, which could be adjusted with setups, of course.
Also for the league racing, competitive gameplay is more rewarding and satisfying, but then again pick up racing generally provides more fun factor.
Altough semi-simulator is also a neat idea, it can go wrong easily if you just slack on coding the physics. For example GRID (Well, that might be considered as arcade) has one of the weirdest physics i've seen, and it's nearly unplayable to play with a wheel.
2: Why would you keep playing a game even if you’ve unlocked all the cars and levels?
It depends, mostly if the game has some replay value I can pick it up again after a break and get excited about it again.
Also for the multiplayer option if that is available, and there is a community playing it still. Driving against other people provide closer racing than racing against bots that have a specific line to follow. Altough if there is a possibilty of coding a learning AI that is aware of players (e.g something similar to what Tim Beaudet made:
https://www.youtube.com/watch?v=BuL6oXqFbuU)
3: If you are slow and you can’t beat the fast guy, what makes you to enjoy the racing with fast guy as you are always lose, is that the pursuit of “victory”?
It depends on how big the gap is on the fast guy. If it's more than 1 second per lap it can be rather predictable, but if there is a possibility of keeping up in some places and actually a possibility of battling, then I can try to pursuit him. Otherwise I'll just try to keep on practising and trying to adjust driving lines optimally to catch up.
4: what do you want to have in the kickstarter as the return? Except for in-game content? What else u wish have?
Well, basically what you can imagine. Keychains, miniature models of the cars, or maybe even T-shirts.
Maybe even an official league as above mentioned, maybe with a cash prize as well if you can raise enough money from Kickstarter?
5: Will you guys enjoy getting your own designed cars or skins, and level design included in the game?
I kind of find it mandatory to have a possibility of designing your own skin, because it is actually rather fun to design stuff.
Track editor would be awesome, altough the minus side depends on the community itself, there may be more bad tracks than good tracks. But that happens in every thing you have a chance to modify.
I think the car editor would have the same problem, altough if it is possible to have a tool that can adjust engine size, amount of cylinders (similar to LFS's tweak tools, but with the possibility to change the type of suspension, centre of gravity, aerodynamic properties, etc.) meaning that they would be coded in the game but customisable when you are designing a new car.
I could find this working if the game itself had a possibility to import the model in-game and setting the car's properties in-game, as well.
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These are my opinions though, I am no coder and lack the knowledge of it completely so I do not know at all how difficult it would be to implement something close to my answers.