I don't have much wishes for the remaining spellcasters, but I do wish we get good additions to Ranger and Rogue.
Ranger
The latest base class revision had a merit: it's highly compatible with the original version. New archetypes can easily apply to both, the only adjustment needed is related to Extra Attack. There is a good chance that the Ranger won't be revised further, because while there are still people who just want a totally different Ranger (which won't be compatible with other versions, and will always ever satisfy a small minority), many others just wanted it to be boosted, and that's what the latest UA variant does.
OTOH all these foolishly endless revisions clearly had a bad result: that Rangers got very little support in terms of options (including archetypes), exactly because it wasn't clear which version of the base class they should be designed for. We only ever got the Deep Stalker archetype, and that's it.
1) Personally I think there is a huge potential to diversify the Ranger by environment (terrain or climate). Now that the revised Ranger is basically environment-independent, it would make a lot of sense to bring environment-dependency back through a multiple-choice archetype, so that there will be Rangers both for players who like and dislike a terrain specialization. A Ranger of the Land could work somewhat similarly to the Druid subclass, offering a list of terrains/environments, but with focus on (a) avoiding hazards, (b) exploiting features and (c) fighting monsters of your favored terrain. The key to make this a widely attractive option, is to design features that actually apply beyond your favored terrain itself (simple example: a Ranger of the Desert could earn protection vs. heat or fire effects > useful also when not in a desert).
2) Urban Ranger sometimes strikes me as an oxymoron, but it can be a nice addition as long as it still has a focus on travel and exploration. So not an expert of a single city, but an expert of cities in general.
3) Planar Ranger or "Horizon Walker" would be another natural concept, for a Ranger that goes even beyond the "range". It could feature abilities to find portals, planeshift, fight/banish outsiders etc.
Sorcerer
The only subclass I'd really like right now is a generic Sorcerer, because all the current archetypes are too specific. I had players who liked the Sorcerer concept, but didn't want to turn into a dragon, nor have random magical effects, nor be particularly desctructive (storm), not tricksy (shadow)...
How about a simple subclass that focuses on the main iconic sorcerer's feature i.e. Metamagic? The base class gets way too few metamagic options, so a new subclass that grants more + additional uses per day, would become the default low-complexity choice for all players who don't want anything more specific.
Rogue
Among the "classic 4" this is now seriously lagging behind in number of subclasses.
Currently we have:
Thief > exploration
Assassin > stealth
Arcane Trickster > magic
Swashbuckler > combat
Inquisitor > investigation
Mastermind > mixed
Thinking about other possible focuses not yet covered:
Spy > basically a con artist, focus on interaction, disguise, infiltration (but not only stealthy)
Artificer > why not? this never works well when designed as a Wizard option, and it's not really supposed to be magical, so why not Rogue?
Plus perhaps a purely "Indiana Jones" style subclass, focused on exploring dungeons, avoiding combat, getting out of traps, solving riddles and finding treasures. I don't much like the names Treasure Hunter or Raider but I can't think of any better on top of my head now...