I think that's a worthwhile goal, and for DM tool or Monsters, easily achieved. However, to be able to do any really worthwhile (as in, worth the work and hassle of a new Edition) changes, there would have to be at least a minor level of disjuncture in player character options (though perhaps fixable with a conversion guide). That's why I hit on the idea of "modular compatibility": maybe you couldn't use the Horizon Walker Ranger with a 6E base Ranger, but bringing a 5E Ranger to a 6E game would work in play.
Looking at Tasha’s, I have no problem seeing updated versions of all the classes that just work better, without making them incompatible with the original 5e phb or other books.
For instance, a ranger that gets a Natural Explorer that has Deft Explorer folded in, Favored Enemy that makes Hunters Mark a freebie and adds another spell depending on favored enemy choice, and switches to prepared casting, would work fine with existing subclasses.
The wording of the bonus spells features would still work, at most needing a general rules in the phb Spellcasting section that notes that if a feature says that you learn a spell and that spell doesn’t count agains your spells known, it also doesn’t count against spells prepared, and vise versa.
Find a succinct way to say that, and that learning a spell means you can cast it with any spell a lot you have, just to smooth over any other issues.
if not giving subclasses at 1st level to all classes, I would also do class variants at Level 1 similar to Khaalis's Light Fighter Variant. I would do one variant for the bard to address the issue discussed in Mike Mearl's Happy Hour with the Valour Bard. I would do a Wilderness Rogue and, maybe, an Academic/Scholar Rogue variant for the Rogue.
Yeah that’s similar to what I’m on about, except I’d just add a feature slot to all classes at level 1, and say, “you gain either 1 skill, tool, weapon, or armor, proficiency of your choice, or a Variant Starting Feature from your class. Maybe weapons could be more than one, but you get the idea.