As it currently stands, Alliance has three races that can be Druids, while Horde have four. There is no reason why Humans should not be able to be Druids at this point. They have been around Night Elves, Worgen and now Kul Tirans enough to have learned Druid ways.
Feral form would fit nicely for a full lion appearance for the mascot of Stormwind Humans.
Their travel form would have to be a horse though, that would be the only drawback...
I really wish that they were planning on adding some new Race/Class combos in Shadowlands. One can dream, I guess.
Addition: If the Night Fae Fauns become playable, they would be a cool race option, I could see their forms being more plantoid like the Sylvari plant cats from Guild Wars 2. Though, there is no guarantee they will become playable, or have druid as a class option, and won't be added for a very long time...