Why not a Design Focused on Shapeshifting? Well, I believe this design idea has been taken by Greymane. He is the one that, as the core design, can alter kit and gameplay based on forms. So I decided to include shapeshifting only as a heroic, instead of being a central part of his kit - though this heroic also alter the way he plays out.
Why not a Design Focused on Summons? IMO, too many summons are boring. Blizzard itself has stated that there are too many Heroes with summoning abilities, and that they'd try to avoid overusing summoning abilities and heroes when possible. So, while summoning still has its part on his kit (with the Nature Spirit, and many Diablo 2 summons included as talents - like Raven, Wolfs, and Poison Creeper), it's not as influential as elemental magic.
Why so Many Elemental Skills? Not being too much focused on shapeshifting nor summoning, elemental druidic magic is what dominates most part of his kit - although not completely. The idea was to bring a more balanced mix of summons, shapeshifting, and elemental, but because of the problems stated above for summoning and shapeshifting, elemental got the better part of it.
Why a Support? Well, why not? We already have our share of elemental-type spellcasters who are burst DPS mages. This is more unique, and, in my opinion, works well as a support character.
Why Melee? Like Xul, he could go into melee using a weapon (maybe a sickle or a spear?). I think the game lacks melee supports, and his kit allows him to be a melee hero without problems, with Nature Spirit and Cyclone Armor adding to his survivability (as long as he has a moderate Health pool to begin with). It could work as a ranged hero as well, but, in my opinion, I think ranged heroes are too overused (except for Warriors, obviously), so when the opportunity to make a hero melee is there, I personally prefer it.
UI Element: As it is getting common for Heroes to have important trait / stacks / specific informations as an UI element above the Health bar, I believe that is the perfect spot to indicate the Druid's number of Ability Charges. The trait icon is not only too small, but can be replaced by some talents.
How does Nature Spirit (Q) Works: It works kinda like Kael'thas Phoenix - you target an area, and then the summoned spirit moves from you until the area. When it reaches the area, it creates the healing over time AoE.
Three reasons I believe this healing mechanic fits this hero: 1) it shares a similar mechanic to the Diablo 2 Druid main healing ability (Oak Sage); 2) it is different from all other healing mechanics for other support heroes, which makes this ability unique and interesting; 3) because the druid has Druidic Versatility, this healing mechanics stops too much healing bursts. If his heal were more like, for example, Uther's Q, he could over abuse healing by using Q-Q-Q on the same target - not only that would make a ridiculously high amount of heal, but would not incentivize the player to use W or E, just Q (OR, a single Q use would have to heal very little, which is not good). In the way it is designed, multiple Q castings allow for healing spread or increased healing duration, but not healing burst.
How does Molten Boulder (W) Works: Plain and simple line skillshot that travels forward until it hits a wall / tree / structure / any other immovable terrain. Deals relatively high damage, but it's not super fast so it sure can be dodged by enemies.
How does Cyclone Armor (E) Work? Target gets buff (move speed, ability damage reduction) and a wind animation surrounds target. When the duration ends, the wind explodes in an area (~ ETC's W), damaging and pushing enemies away. I think ability damage reduction is an interesting addition for this ability for 2 reasons: 1) it's a homage to how Cyclone Armor works in Diablo 2; 2) since his healing mechanic is over time around a set area, I imagined this hero would be very weak against heavy AoE burst damage like Kael or Jaina. Cyclone Armor helps mitigate that weakness.
How does Arctic Blast (R1) Works? Firstly, this heroic taps into the druidic versatility, granting him additional Ability Charges. If the player wants, he can never use the heroic properly, instead benefiting from using basic abilities more often. Or, he can use the heroic more often but less basic ability uses. The ability itself should not be very strong - it should be roughly the power level of 2 basic abilities. It's more closely to those heroes with constant use or low cooldown heroic, which feels more like a new basic ability than a heroic (like Hammer's Napalm Strike).
When used, the Druid targets an area (big cone - greater in length and radius than Jaina's W), and after a casting time (something akin to Falstad's Hinterland Blast), the frost cone is released, dealing damage (not super high) and slowing enemies a lot, and granting shields to allies. When first activated, winds fill the area in a visible way, in order to indicate to both allies and enemies where the ability will take place; after the casting time, the area is filled with ice.
Positioning is super important: you want to hit as many enemies and allies as possible, but sometimes you'll have to make a choice between granting lots of shielding or damaging and slowing. It's a very versatile heroic not only in its basic usage but also for interacting with the trait.
How does Werebear (R2) Work? Simply, you turn into a Werebear, gaining huge self-buffs in lieu of not being able to use your abilities and talents. I opted to include Werebear, instead of Werewolf, mainly because Greymane is already an werewolf.
Since the Druid's main abilities and talents are focused on healing, buffs, pushes and damage, I believe it adds to the Hero's versatility if the Werebear offered a new strength, so the ability is designed to make you more tanky-ish. You gain greater Health, the ability to regenerate your health, and a short-cooldown AoE slow. Although it doesn't make up for a complete solo tank, it certainly allows you to fill the role a little bit.
The way this heroic is set up allows for extra versatility. You can keep switching back and forth from human to werebear as you seen fit, until you spend a total of 15 seconds in Werebear form; at this point, the cooldown begins and you can't switch again until the cooldown ends.
When in Werebear form, your Q is replaced by Stomp; your W becomes Shockwave if you get this talent at level 20; your E icon instead shows the remaining time you can spend in Werebear form; and R is used to switch between human and werebear.
Heroes of the Storm Build Guides
Second thing I love, is that this is Druid. I played long time in Diablo 2 as Werewolf Druid and I just have big sentiment for this character. Plus I love his design, he's looking very unique and characteristical. I'm sure he would work epic as playable Hero of the Storm.