I doubt you can find an expression which means anticipation and bears the fifty-fifty chance connotation as well.
You can use toss-up with a verb that expresses anticipation. Cambridge says
If you describe a situation as a toss-up, you mean that either of two possibilities is equally likely:
- It was a toss-up who would win, right to the end of the game.
OxfordL labels it informal and spells it in one word:
a situation in which any of two or more outcomes or options is equally possible or equally attractive.
- In the end it was a toss-up between the mussels, the crispy prawn parcels, and the smoked trout.