I played with the idea earlier, and came to the following conclusions.
1: it doesn't feel like turning into an animal. It feels like turning into a mutant.
2: it's too much to add to a full caster. It would be nearly a class on it's own.
1. That's subjective, but that is all in the mind. As long as it is written in a way that supports the fantasy, it shouldn't be a problem.
2. Enthusiasts of Advanced Wildshape (only available from the Moon Druid subclass) want a fully functional, tough, capable, melee combatant that can fulfill the fantasy of ANY beast in the game. That is tricky to do with a class that already gets so much full caster functionality. I honestly don't know if there is a way to balance that.
Because we all can already reskin anything into what we
want, all a Wildshape subsystem really
needs to do is at least allow the Druid to turn into a beastform they prefer that
works like that beastform, and every rule for that class and subclass
needs to fit in the space of that class and subclass, including the forms themselves. If it can't fit completely in the PH, it can't work. A player class should never require looking at any other book. It needs to be self contained in the PH. It needs to work for new players who only have the PH.
I played the 2014 moon druid and it was broken because of the ablative hit points. It also didn't allow me to be a beast I wanted to be. I had to select undesired forms just to be functional. The druid should not have to turn into a dinosaur or mammoth to be useful at high levels. For 3 of my druid concepts, I am only interested in a small number of beasts, and I want to be capable at those forms at every CR. The game needs to do that without flooding other books with 6 different stat blocks for each beast I want to focus on.
I played the most recent UA moon druid at high level, and:
- I could not fulfill the fantasy of wildshaping into a preferred beast I wanted to embody. I had to pick something else to feel functional. There just wasn't a good variety of choices to fulfill my preferred theme.
- The different monster stat blocks available were not balanced to the level of the adventure, or against each other. There was wasn't a good variety of options for effectiveness because they are balanced to Monster CR, not player effectiveness.
- This boils down to that fact that it could not fulfill most of my different druid fantasies.
The druid concept has the same problems as the psion concept. It means too many different things to too many different crowds, and it can't make everyone happy. The additional problem with the druid is that they already exist in a broken format, and the designers have the unfortunate responsibility of fixing it while also making druid fans happy. I will give them a well-deserved slow clap if they figure it out.