In Pathfinder and D&D 3.5, some classes were clearly better than others. If you put a level 8 party together with two martial characters and two full casters, you'd find that the Fighter and Rogue were mostly just there to make the enemies hold still while the Wizard and Druid killed them, and that on the days that they failed to show up, the party still did fine, but took rests slightly more often. If the Wizard didn't show up, progression halted while the group tried not to die in every significant encounter.
Fourth Edition (not that I played it that much) apparently fixed that problem, but the cost they payed was that the classes all felt kind of similar - they all gained similarly complicated abilities at similar levels, and your fighter was casting sword attacks in all the same ways the wizard was blowing things up.
5e feels much better to me - the characters are very distinct, the abilities are clearly different. The power levels diverged a little again, but not as far as before, and the martial characters stayed interesting to play. There's one class though that still provides a real balance problem - the Moon Druid. Now, I'm not here to tell you that the Moon Druid is overpowered - the truth is that it loses a lot of its effectiveness from levels 11-19, where it gets outpaced on every front but the ability to lose hit points safely. But Mooney is painfully over-effective at low levels, and deserves to contribute a bit more at the higher levels. And his capstone ability is simply silly currently (a new wildshape every turn is like being hooked up to a fire hydrant of HP).
So I started exploring the homebrew scene for Wildshape to find approaches that seemed more balanced than the rules as written. Credit should go to Easy_Lee for the bulk of the mechanic described here - I only made minor tweaks from his system where they seemed natural.
When preparing spells, you also prepare wildshape forms for the day. You get 1 prepared form at level 2, and additional forms for aquatic, flight, and possibly elemental forms at the appropriate levels (the prepared forms in those slots are allowed to have swim speed, flight speed, or be elemental, but are not required to). Moon druids may select one of their prepared forms to use the improved CR restrictions - the other forms must abide by the normal druid CR requirements. At level 9, Moon druids may select two prepared forms to use the higher CR limit.
You may not use 'Multiattack' in any form until druid level 5.
Your forms can be entered and left as an action (bonus action for Moon druids), and have no time or frequency limits. Their HP are persisted for each form, and are recovered like player HP (spend their HD to recover health during short rests unless unconcious, recover HP and dice during long rests). Conditions are also persisted, but are removed with even a short rest.
Having a form's HP reduced to zero reverts you to your normal form, deals overflow damage, and also stuns you until the end of your next turn. A form with zero HP cannot be used (and its HP cannot be recovered) until a long rest. (I have reconsidered this, and removed it from the final version).
Forms with HP less than 4x druid level may use that as their HP instead.
Physical skill bonuses are optionally recalculated using your proficiency and the shape's stats. If the beast has a higher skill bonus than that, you may keep theirs instead.
Moon druids may choose to use their spell attack bonus instead of the form's attack bonus, and may choose to use their spell DC instead of the form's ability DCs. Note: you can leave this change out to reduce complexity, or possibly make the option available starting at level 8 or 9.
If a Druid of the Moon casts Barkskin on themselves, they may choose to cast it on one of their forms instead, even while they are not using that form. If they do so, then the spell will apply only when they are in that form, and only time spent in that form will count toward the duration of the spell. Concentration is still required in and out of that form. This compensation is important because druids care about their form's HP now, and have no other way to boost their armor. A possible (simpler, though less thematic) alternative is to add Mage Armor to the druid spell list for Moon druids. (I changed this to be based on spell DC instead of a spell, so it will scale properly)
New Moon Druid Capstone ability: As a bonus action or while using a bonus action to change forms, you may restore all forms to full HP once per short rest, and may prepare new forms in each slot as you do so (you may not change a form that you are currently using, though you may change a form you are changing from or into).
New Land Druid Capstone ability: While in wildshape, you may provide material components to cast a spell if they are merged into your form, you may treat your body as a druid focus, and you may spend your current form as a level 5 spell slot (reducing it to zero hp and immediately leaving the form when the spell is cast). This ability may not be used again until you have taken a short rest. (The new wildshape mechanic makes the old Druid capstone irrelevant - this one feels more thematic for Land druids anyway.)
Aside from the obvious effect of keeping their ability to soak up damage in line with other similar characters, there are a few points to make here about the impact of these changes. First, the amount of preparation is reduced - druids only need to have handy stats for the forms they are preparing (and remember, they don't need to have seen a beast to use it, which saves DMs from having to work a mammoth into the campaign to give proper scaling).
Secondly, it is very freeing not to have to worry so much about durations - deciding how much time has elapsed between important events is usually handled via DM fiat, and resting such a tactically important resource on that decision always gave it way more weight than it deserved.
Finally, moon druids in combat will need to take damage seriously - getting knocked out of combat form in a melee has serious consequences, and form HP is real and won't be magically restored every short rest. Druid dominance at low levels is reduced by disallowing multiattack until some other classes are able to get an equivalent ability, and flexibility is dramatically increased - being able to swap back and forth between forms means that the druid doesn't need to sacrifice his utility in case he needs another wild shape later. Being able to shift in and out of a form without wasting resources opens that ability up to be used in noncombat situations, both for roleplay and just play (restricting them to prepared forms keeps that utility from getting out of hand). By far the biggest pain point there was that a druid that has shifted couldn't afford to shift back to talk, lest he waste an important resource - we can get rid of the standard "I whine at my allies until they figure out what I'm trying to say" conversational tactic almost entirely!