I'm pretty sure there's been hints of other continents before, so I can deal with this coming up. Unfortunately, this is Mashima, so I have no expectations of this working.
If Mashima was a great writer, I could see things working out this way for the arc to actually go. Over time, between all the missions and Dark Guild attacks (along with the tournament), the guilds have come to respect each other more and while still being competitive, aren't adverse to the idea of working together. We had the Oracion Seis arc with certain guilds working together, and the Avatar arc could've had Sabertooth helping out, showing that they are willing to help out as a guild, showing the change. Instead of Fairy Tail having gotten back together, the main group found out about Makarov going to a different continent, with rumors going around about another war breaking out. The other guilds decide to have their masters go to check things out, which with the masters being gone so long, tension is rising amongst everyone. It's decided that the only option they have is to use the little power they have to amount an assault on the kingdom, as if the kingdom attacks on their land, there will be many casualties among citizens. This would allow the last three of the great mages to come and show off their power. With the remains of Fairy Tail allowing all the guilds to work together, they go off for an assault.
We could also have various Fairy Tail members appearing to help out, allowing them to remember their connection with the guild. When they end up saving Makarov, it would allow for a truly celebratory reunion of the guild. There would be more impact because there would be more time of the guild being disbanded, and set in the idea of an incomplete Fairy Tail going on this kind of mission, where the whole guild was helping with Tartaros.
At most, we can hope for some struggle for the members. If anything, I see Fairy Tail making some victories, but the rest of the work is done by Zeref and Acnologia, destroying everything else themselves to hype them up.