Rowlet's either staying as it is or fully evolving. Dartrix is by far the least popular of the Rowlet line, I highly doubt Ash will end up with it. I also don't think Decidueye is going to receive the Chesnaught treatment, because unlike Chesnaught, the Japanese actually like Decidueye. If Ash doesn't get it, then they're probably saving it for a future, likely anime-exclusive rival.
Eh, to me, Litten fully evolving was obvious. Incineroar may be the least popular of the fully evolved Alolan starters, but it is a Creator's Pet due to the relation to Tiger Mask.
tl;dr: Sometimes, the Pokemon that was caught first evolves first, sometimes it doesn't. There is no set pattern. Call me delusional, but I still think Decidueye for Ash is a possibility, and I'm going to believe it until one of two things happens. Either an important character with either a Dartrix or Decidueye shows up or Rowlet indicates it doesn't want to evolve. If one of those two things happens, I will gladly eat crow.