I say it shoould be a Monk. Spiritual Energies for abilities, and unarmed combat.

Sentinel/Ranger. Spellcasting and ranged weapon specialists, depending on spec. Although making them healers would take a little bit of tweaking, but like Tyrande being both an archer and a healer, it should work out.

Dear lord...I look at some of the comments below and I lose hope...Only allow it to be 1 race, are you serious?

Completely agree with Lach, we should stop using the phrase "Hero class".

Whatever the next class (0r race) is, it will be tied in to the theme of the expansion. Blood Elves / Dranei were tied to the Outland story, and Death Knights were tied to the Northrend story.

This makes it very unlikely that we'll see Demon Hunters, since BC was the demon story, and Blizzard is unlikely to revisit that. The same goes for players wanting playable Vyrkul or Tuskarr - if they weren't introduced in WotLK, they're never coming.

My guesses: If the next expansion is Emerald Dream related, expect a dream/illusion themed caster-healer class, druid-like, but different enough not to be druid++. If it's a Maelstrom expansion, expect playable goblins.

I agree with Aler that a dream/illusion powered healer would most likely be the next hero class, assuming an Emerald Dream expansion. However, I think Bliz might avoid a back-row healer, instead opting for a class that can DPS and heal well at the same time, without respeccing or changing gear.

As for what it's called, obviously Archdruid presents many issues and doesn't seem likely. I don't think Bliz will feel confined to the names of hero classes from WC III, so Keeper and Warden are good guesses.

Then again, it could be a monk-type deal (unarmed spec ftw!), with lots of buffs and healing abilities.

I think some sort of tech based class that uses rogue like energy that quickly recharges.
DPS with a gun, uses tech based devices that drain energy in return for buffs/debuffs and healing. The devices could possibly appear like totems or alternatively maybe use the devices and you get in the machine sorta like the spider bots in gnomerigan(a more mechanical equivalent of tree form).
The class would be available to nelf or tauren because they are hippies who don't like technology(they get druids! quit complaining :P).

I would look no further than previously created classes from the Warcraft RPG for hero class ideas. Two candidates stand out brilliantly for an Emerald Dream and Maelstrom expansion.

Runemaster - a monk-ish healing/DPS hybrid using the DK's established rune system. They are all about buffs, personal and group and could dish out steady heals without ever having mana issues or self buff and crank out some dps. I am actually very surprised this didn't also come out with wrath considering all the ley line and rune lore they have painted all over that expansion.

Tinker - Engineers gone wild! This steampunk combatant can be tailored to a variety of roles and abilities.

I'm leaning more and more to the next expansion being the sea, and so I'm leaning more and more to the next Hero Class being Shadow Hunters trained from heroes of all races by the Zandalar.

Tinker!