My thoughts on the Playtest Ranger:
Expertise. I think this is fine both thematically and in power level. And it does not hurt backward compatibility.
Favored Enemy. Give the Ranger his choice of actual Favored Enemy. Instead of altering a spell, (Hunter's Mark) make this a non-magical ability that the Ranger has. Then it can't be dispelled or counter spelled and doesn't require casting. If you say that actual favored enemy is always considered affected by it, then the choice of enemy has value and the Ranger can still put it on someone else, and simplify. Just say add one damage die to the attack. i.e. Longbow does 2d10 instead of one vs. the target.
Spell Casting. I think this is fine. Adding Cantrips and making the ranger a prepared caster works, but give classes an actual spell list both for ease of use and to avoid missing spells that the Ranger should thematically have. You could leave out spells that they shouldn't have and to preserve backward compatibility. Give Rangers a spell focus.
Fighting Style. I think this is fine both thematically and in power level. And it does not hurt backward compatibility.
Sub-classes, deserve to be addressed individually. The ones presented are not great, and not terrible generally.
Feats, and Extra Attack work like they do for other classes and they are fine.
Roving. This is a strong (though situational ability). Much more useful than the 5e counterpart but I think it's fine for a 7th level ability.
Tireless. The THP are unneeded to me. Just make ordinary exertion from travel, labor etc. not cause exhaustion to the Ranger. As a side note, I think we can stick to 5 levels of exhaustion. I like that the playtest version is streamlined but 6 levels is plenty to cause death.
Nature's Veil. Instead of altering a spell, (Invisibility) make this a non-magical ability that the Ranger has. Then it can't be dispelled or counter spelled and doesn't require casting. Just say, when the Ranger hides in a natural setting, he becomes invisible until revealed.
Foe slayer. This is good, and it should be for an 18th level ability, but simplify. Just say add one damage die to the attack. i.e. Favored enemy now makes a Longbow 3d10 instead of 2d10.
Epic Destiny. I like this idea a lot but make the choices truly Epic. It's 20th level.
My 2 cents only IMHO. I know some just don't like the Favored Enemy at all because it's so situational, but it's easily fixed by an ability that can be targeted if your actual favored enemy isn't present in the fight.