Well, as the title says, I think it would be good to add a feature that simply allows you to turn off the engine. I loved this simple feature back in RoR. It added a sense of realism IMO. I think it would go well with the temperature and engine damage mechanics as well. Too much engine damage/overheated engine will prevent you/affect from turning on the engine (damaged gasket, damaged piston rings, block damage, etc). To couple with this suggestion would be more SFX. This includes start up sounds (bings, bongs, and engine start ups) and even unique sounds to different cars. In addition, the fact that all the cars sound the same kinda bothers me. I know there are mods to which I can change the sounds of the cars, but I think it would be much easier and convenient if the sounds were made official. A good example is what you did with the Ibishu Covet. The engine sound that you made for that car was perfect for it!
They're working on it. The SFX are slated to be completely redone at some point, with the current sounds being placeholders, and the ability to turn the engine on or off has been suggested many times in the past and stands a good chance of being a thing at some point. Also, in the future it might be a good idea to use the search bar to make sure that your idea hasn't been suggested several times in the past. Some of the older members here have ran out of patience for this kind of thing and can be a bit harsh about it to newer users.
