For one, Malfurion. Introduced as a sort of regular albeit powerful/important character in WC3, turned into pretty much a Demigod over the course of Knaak's books, Wrath of the Ancients and Stormrage.

Broxigar, although a nice character somewhat, being the only mortal to hurt Sargeras.

Broxigar's niece we'd never heard of before in Stormrage, being manipulated by the all-powerful (or not quite) Malfurion into releasing him. Why her? She gets thrust into the center of things, maybe not necessarily powerful but important, rising from obscurity. She's just a bad character imo.

Lucan Foxblood, a Human DruidSomethingWhatever, being able to jump in and out of the Emerald Dream and now being under the tutelage of the only Tauren Archdruid, Hamuul Runetotem.

Pretty sure there's others, those are just the ones I could think of off the top of my head.

Rhonin raised an army of freaking intelligent raptors!! and was chosen by a mighty red dragon to be the only one to free his mate and mother of all the red dragons from a hostile environment and a fortress full of orcs... despite not being outstanding in any major way before. He also got the girl, and now he's the leader of the Kirin Tor .... yeah...
Somewhat of a powerful character, mainly Knaak's typical Mary Sue though.

Krasus is a very old red dragon, him I don't mind being powerful.
Malfurion, despite also being old (young compared to dragons though) is still only a mortal so the stuff he did in War of the Ancients and Stormrage is just ridiculous. I get it that he is powerful but what Knaak had him do.... argh...

I'd love to also pin Med'an on him, he certainly fits his MO but couldn't find a source confirming that.