There's a couple ways I see this going
1) Zeira turns out to be like Mavis is in the series — A astral projection that only a select few may see. Such as only those with the Fairy Tail Emblem being allowed to see Mavis, Mavis is unique in someway that allows her to see Zeira.
2) Zeira turns out to be a fairy
Now the reason I definitely believe she is not an illusion, is because we have seen Zeira hugging Mavis, and contact has been made. However, it is clear that Mavis's illusions can not interact physically with the real world. So it is safe to remove the idea that she is simply an illusion.
To be honest, I always thought Fairies would be the last thing to be seen in the series, being just like the Dragons. I feel as though Mavis's actions lead her to become a Fairy, which is what will really help defeat the final villain (e.g. Zeref or Acnologia) This sorta leads me to think Zeira is a fairy. But, I actually think that it's gonna be something like Mavis has always been a fairy, but she lost her memories and got amnesia and Zeira is just an astral projection only a select few can see.
I ramble though.
TL;DR — Zeira is an astral projection only Mavis sees (for now) and Mavis is a Fairy