I'm pretty sure that every class has something that is unique to it, otherwise we'd have a much smaller list of classes. I think the ranger has too much history to have been made into a subclass
Ranger, Paladin & Barbarian all started as Fighter subclasses. In the case of the former two I wish they'd gone back to that. I like how the 5e Barbarian worked out but it would have been ok as a subclass too. Ranger & Paladin IMO turned out very badly in 5e, being spell-based from level 2 I feel is completely inappropriate, and this is probably my biggest dissatisfaction with 5e.