First off, I suggest that there be an eventual druid pass. It's a great class with a lot of flavor (something missing from several other classes) and variety that can be really fun to play.
I'm not going to touch on melee druids much as I haven't played one since druids were new. Caster druids are really in a solid place, they just need to be brought up to current levels of play. First off, get rid of the casting penalty for animal form. As druids are now, you have to really dedicate yourself to being a caster or a melee, there is no viable hybrid. So why not let casters run in animal form? It would be strictly for flavor and not give any real sort of advantage at all in the current game. And at the same time, lets see some run speed hikes in animal form. In reality, Bears can hit nearly 40 mph for short sprints. (I've seen them quite a bit in the wild, and it's truly incredible how fast they can move!) And wolves can run the same speed for quite a long time. Druids are one of the few classes that get no speed boost at all, and in the modern zerg fest, it's really hard to be viable when you're always bringing up the rear. And it makes no sense. A bard or a monk with no magical boost can naturally run quite a bit faster than a 40mph animal? The fastest man in the world runs about 28 mph... and he's the top of the heap.
If a third tree is in the cards, I suggest one that deals with balance, alignment, and healing. This would be a nice tree to dive into for either a melee or caster druid, and would create a new healing class, hopefully with hot slas. It would also be nice to have more than one spell, sla, or ability based on alignment and negative power. Word of Balance is a really cool spell, but that's it for alignment. FoD is the only potentially useful negative spell. It would be terrific to have some negative energy debuffs, maybe harm type spell for healing undeads, etc.
Improvements in the caster tree: There's a lot of broken and useless junk in the caster tree. The capstone critical hit on season change is bugged, (and while nicely imaginative, not really that useful in epic/legendary content where a good caster will have >50% crit chance at all times) The +2 dcs to conjuration and transmutation power are baffling as they help very little. Most transmutation spells are buffs and the few conjuration spells that require a DC check are very underpowered, slow, and rarely used. Give us evo DCs, and maybe necro dcs. I can get my necro dc into the 80s, and with debuffs, it's very useful on low fort targets. But it's expensive to build for and to use, and is completely useless against mobs with any kind of fort save. A +2 would be very helpful to make FoD something slightly more useful. (Most casters don't even bother now) And there are several enhancements like the tier 4 Autumnal Susurrus that are baffling. Acid spells (there is one, SoV) negative spells, (there are 3, FoD, poison and contagion) have a max 40% chance of applying shaken to targets. Really not a bad deal, but the very limited range of spells that apply the condition is silly. And FoD is potentially an instakill, so what's the point of applying shaken to a corpse? This should maybe combine with beguile, although as far as I can tell, when Beguile was nerfed, it was hammered into pointlessness. Instead, lets see a little bonus for critical spell damage. There is currently nothing that's inherently druid, and the only way to get this is with epic/legendary feats.
And although I don't claim to know much about melee druids, the fact that so many of the attacks in animal form are spells is really annoying. Ever try attacking a beholder? When out of sp? I don't have an issue with an obvious magical attack like a cone of cold or something, but bites and claws?
The pet is barely useful in heroic. I rolled a druid back when they first came out and the pet was very useful at low levels. It was nerfed (probably rightfully so) but they should become unnerfed past level 20. Pets have no epic destinies, legendary feats, and only a couple slots for gear. They simply can't keep up at high level. They need real defenses and real offenses. Much of their tree doesn't work either, and rather than getting clever ideas that don't work, how about just allowing them better saves, better AC, more prr, and more hp? Then at least they wouldn't be such a liability when they die and take hp and sp from you. And it shouldn't be that hard to have a couple of items just for pets that give us 4 or 5 slots for augments so we can customize them a bit. Boost a couple of stats, add resistance, etc. Then they might become more useful, or at least not a problem.
I know pet AI is a huge factor, but one thing that would help is if they stay put when the flag is planted even if they pull a lever. For example, in Prison of the Planes, I would love to use the pet to pull the lever, but most of the time as soon as the lever is pulled he runs into the force barrier and dies pretty much instantly. And PoP only has one shrine, so losing sp in there is a real problem.
I hope this is not taken as negative. It is meant to be constructive criticism to improve a really fun class to true legendary status. I know it's more fun to develop new classes and stuff, but many players really appreciate updating old systems too.
Thanks for reading and feel free to comment or criticize my ideas.