Character Concepts you wished would work in D&D?

[Dr. Cliché]

I understand the complain about Moon Druid, so yes, that would be nice. Though I also could see an option where you get stronger self-alterations (natural attacks, tentacles, wings, more AC), or stronger forms (humanoids, giants, dragons, with a HD max, or just a few fixed forms).

I think there's a lot that could be done with shapeshifters, to be honest. There are so many different versions in both myth and popular culture.

At the very least, it would be nice to see the number of shapeshifting-classes expanded a bit. The Warlock seems like a prime candidate for a shapeshifting subclass (and/or a shapeshifting item - like the enchanted pelts or belts skinchangers use to transform).

Ranger seems like another possibility, to keep some of the nature theme of the druid but focus it more on combat rather than spellcasting.

I also miss that in the caster department. It's really a shame 5e didn't continue on the path that late 3.5 took, with the beguiler, warmage, dread necromacer - powerful, versatile spellcasters but with a very specific theme, and different strengths and weaknesses. The generalistic approach leads to too much the same spells be prefered (fireball, fly, invisibility, shield, absorb elements, banishment, misty step, animate object, wall of force, etc. etc.) regardless of subclass / school focus.

I'd definitely agree there. I think many of the current wizard subclasses are just too broad or have too little interaction with their supposed theme. As an example, what benefit does a Transmuter get that makes Transmutation spells more appealing? If anything, it seems more like a weird alchemist than a wizard focused on Transmutation. The Necromancer is a little more in-theme, yet the only Necromancy spell it actually supports is Animate Dead.

I think a big part of the problem with the whole subclass mechanic is that there's no sacrifice involved. With a class you can add additional features and balance it by removing existing ones, in order to make the class much more focused on its theme or specialisation. But with subclasses, there's generally no removing of core class abilities, so you're much more limited in terms of what you can add, as you can't touch the base class.