The only reason I have Bryant as a tier 3 instead of a 2 is because he does not have any MLB playing time. You could make an argument for him to be tier 1 right now. I made the list thinking in terms of the OSFM project, everyone in tier 1 & 2 already have MLB playing time and have a decent chance of being on the default roster that launches with the game.
I will be updating and reorganizing the list later on today. Let me know if you have any suggestions. Also, if you come accross any good Cubs top 20 or top 50 prospect articles, please post it here. I feel they give a really good idea what players should be included in the OSFM sets.