It is worthwhile to standardize the amount of design for a subclass across all classes. Schedule the same levels for the subclass features in a character advancement table, so they can apply to all classes.
The UA Ranger gains spells at:
levels 1 (3), 5 (7), 9 (11), 13 (15), and 17 (19).
The 2014 Fighter and Rogue gain spells at:
levels 3 (4), 7 (10), 13 (16), 19.
A future UA Fighter and Rogue can easily gain spells from level 1:
levels 1 (4), 7 (10), 13 (16), and 19,
for a smooth spell progression every third level.
It is convenient to coordinate the levels for the subclasses with the same levels when Fighter and Rogue gain spells.
For example, going by the 2014 Fighter-Rogue, all classes can gain subclass features at levels 1, 7, 13, and 19.
Possibly schedule the subclass features to coordinate with levels 1-and-4, then 7, 13, and 19, to help flesh out the mechanics early on for the subclass concept.
Alternatively, coordinate subclass features for all classes with the spells at:
levels 1, 5, 9, 13, and 17.