The whole business model for a free-to-play game like Heroes of the Storm is to continually get content out. While some of that is purely cosmetic stuff like the yes-goddammit-I-bought-it Star Lord Leoric skin and mount, it also means a continual release of new heroes. New Heroes get to shake up the meta-game, but are also just plain fun. If you've been reading the last couple entries on this blog, you'll notice how infatuated I am with the newest addition, Xul the Necromancer (seriously, he's just so damned fun.)
Thankfully, Blizzard has three huge franchises with tons of characters to pick from, with Overwatch coming relatively soon as well, and theoretically they could delve into older stuff (though the only example so far are the Lost Vikings - a niche Hero(es) to be sure.)
Anyway, let's go by franchise:
Warcraft already has the highest representation in the game, though that's understandable given that it's also the most detailed and fleshed-out world, with the most Blizzard titles attached to it (if we count expansion packs as separate titles. Or even if we don't.)
There's a pretty decent spread of the playable races in WoW, but here are some people I'd consider adding:
Baine Bloodhoof (or Cairne)
A Faceless One (maybe create a name, like they did with Lunara.)
The Monkey King
An Ancient (the tree ones, not the Wild Gods, and give it a name.)
Far smaller in scope than Warcraft, there are some characters who are begging to be added.
First, possibly the male/female version of existing Diablo III classes.
Female Witch Doctor
Male Demon Hunter
Then you could look at other Diablo and Diablo II classes:
Barbarian? (though that might just be the Diablo III one.)
Then there are established characters:
Deckard Cain (there's an awesome theoretical mock-up for him on Carbot's youtube page.)
Abd Al-Hazir (please! Granted, his abilities would probably all be "get assistant killed horribly and then act as if you're the wronged party.")
So lots of options there.
Starcraft has fewer characters to choose from, but there are a handful.
And of course any units can be given a name - we've seen that with Lt. Morales and Sgt. Hammer.
We won't likely see Overwatch characters until that game has launched, but I'd expect to see Tracer as the probably first one, then perhaps Winston, Reaper, and Windowmaker.