Yeah, the OP is confusing. What determines whether you have to use a keeper slot or not - whether the player is ever promoted, or whether they lose their prospect status (130IP/50AB), or something else?
Obviously you should take the elite prospects, even if they are likely to get enough PT to make them count against your keepers (assuming you have more than just a few keeper slots). Players like Profar, Taveras, Myers, etc., have enough professional ABs and scouting reports behind them that more than likely guarantee their success at the next level. I'd be more likely to spend a pick on one of these heralded prospects who may lose eligibility than attempting to determine who may be the #1 prospect for the following season.
But with that being said, in response to your question - considering most of the above guys losing eligibility this season, my top 5 hitter predictions for next year would be 1. Bogaerts 2. Baez 3. Buxton 4. Sano 5. Russel (6. Dahl).
With pitchers, Jose Fernandez may be the #1 pitching prospect this time next year on many lists, but he has yet to develop plus secondary pitches to accompany his plus-plus fastball. My preference would be 1. Cole 2. Walker 3. Taillon 4. Fernandez 5. Giolito (dark horse).