Could there be SP mode without frames? you know, like a sandbox.. and you could end the replay anytime you wanted.. and you'll only adjust frames per click. it could be useful... you know, when you don't have enough frames to finish your super awesome replay or something

you can do that in notepad

Make a replay and start out with 5000 frames > Finish the replay > Open it with notepad > edit out the extra frames

Creating a whole new Single Player doesnt seem ideal for something that just takes a little extra effort.