Any explanation for why Aeris is there is going to sound far fetched. I think we can rule out coincidence.

There are several possibilities:

a) Ivalice is the same Planet as FFVII at different times. That would mean that the Lifestream could be involved and that Cloud traveled through time, either future or past. Most likely that Aeris would be a reincarnation if this was the case. However, the geography and city names don't match.

b) Ivalice is on a different planet but at the same time. It would be hard to explain how there could be two Aeris' at the same time, unless they were completely different people. Possible but unlikely.

c) Ivalice is on a different planet at different times. That would account for how Aeris could exist on both, but how would her soul travel between planets if the Lifestream stays put?

d) Ivalice is a parallel dimension. So people could exist in both in parallel planets and be unaware of each other. Very popular with science fiction writers. We only see a small fraction of the world in Tactics. There are other countries we don't get to visit, so it doesn't rule out other parallel people.

Of course she's really there because the game designers wanted a cute re-enactment of the scene where Cloud meets Aeris. If she wasn't included there wouldn't need to be elaborate explanations. :rolleyes2