I feel like they have very nice selection of cars and tracks so my choice would be #3 whether it be "endurance" or any other form. Until they have some type of racing, even rudimentary like in AC, then they won't be a serious competitor to anyone. I'm not talking about MP either. Just a plain old race against someone besides the clock.
After that, having some car sets that work for various racing series would make this a very credible alternative to other games. I would prefer GT and LMP cars myself but I'm happy with what they have at the moment. Once they are up and running with SP and MP racing then I think a focus on certain types of real racing within the R3E game will make it very appealing to various groups of Sim racers. Endurance racing would be great for many but it seems that you need 1& 2 in your list plus driver swap to make it work. If they are first to the plate with that then it will make certain groups of sim racers very loyal to the game. Same goes for GT2/3, hill climbs, rally or whatever as long as they make it possible within the game. DTM is a good example of niche games within a game, although having it stand alone and not integrated is a serious problem and a very real irritant to many. I understand someone else paid for it and wants to keep it separate, but it was still a bone headed move in my opinion if they are trying to attract long term customers to R3E.