There are already fairies in the game; of course, you could add a second faction of pixies, but why? There are many great ideas for new factions right now, and maybe the idea of new pixies is simply excellent, and even better than pirates, imperials, and lizardmen, but I doubt anyone will take on the Winx. Perhaps in 10 years, there will be an opportunity to add 100 cities, and creating a creature will be a piece of cake, but that's definitely not now. Turn to the WOG; they will soon be able to easily create creatures, but they have already messed up the third game.
And here are the Winx fairies themselves:
Don't confuse them. In English, Winx are fairies, and small fairy pixies are pixies. And the creatures of the 1st level of the Forest Alliance were originally called "pixies." And fairies are a much higher rank; Winx are definitely hero-level.