Bryan most likely would have done something with Scott, since he has stated time and again he thought he was a great character and favored Marsden. Likely it probably would have been a huge Wolverine movie too, not to say that the real DP is without its Wolverine. It [probably] would have been the love story of Scott and Jean, while Logan got all the hack and slash action the audience could handle. Scott probably would have led the team much like in X1, probably would have had the emotional scenes between him and Jean and then possibly some sort of fight between him and the main villian or whatever. Cyclops role in Singer's X3 likely would have been about the size Storm or Jean had in X2. People act like Singer marginalized Scott somehow, which has to be the dumbest thing I ever heard. The first movie his character was probably only second the Wolverine in terms of X-Men, and maybe larger than Xavier. He had great scenes in that movie, and they actually revealled he was more than just some uptight jerk. Even in X2, despite the somewhat unwarrented openning scene with Wolverine, he had a fair amount of screentime considering in the graphic novel he had even less. And again Singer gave him a great moment on the X-Jet which was quiet deserving of his character.
Furthermore I don't know exactly what role you want Cyclops to have had in Singer's X-franchise. Did you want him to have a ton of lines: guess what? It would be incredibly out of character for Scott to be talking all the time. He DOESN'T DO THAT. He talks to very certain people (Jean, Xavier, Warren and Beast in the comics) and the rest don't get to see anything past a stuck up exterior. He is and will always be a hard guy to get to know, especially for someone like Wolverine, Bobby, Mr Lorrio, or anyone else for that matter. The fact that Scott's most revealled and longest lines were between him and Xavier and him and Jean in X1 and X2 is actually quiet in character for him. In fact it's exactly how you expect Cyclops to act. The fact that Jean chooses him over Wolverine and the audience is befuddled is probably more or less a correct response, because no one else but Jean known what goes on underneath Cyclops big head of his. The lines in the turbine room, in museum, the medic bay in the X-Mansion with Xavier, this silent brooding Marsden does so frequently, that is Scott Summers.