These all sound like really good ideas. c:
Maybe for our first arc, we can start with something that's not as big as the Tartaros Arc but still more important than the Macao and Daybreak Arcs.
Maybe there can be a legend about a magical item that grants one's deepest desires and recently archaeologists found said item and tried to use it, but it didn't work. They post a job request for some wizards to check it out because it is a magical item, but when the wizards get there the archaeologists are missing, the place is ransacked, and the magical item is missing.
One of our dragonslayers could recognize a scent of someone from a dark guild that they clashed with not too long ago, so the team goes after them to get the item back. They end up running into a less powerful member of the guild and take him/her/them down, and that person ends up spilling the guild's secrets. The magical item didn't work because a sacrificial ritual needs to be made, and that's what the dark guild plans to do in order to get their wish (which could be some kind of evil wish like death upon the people who can't use magic).
There's a bigger twist though. The sacrifice need to be a human with noble blood, and the guild has already captured some noble's daughter/son (or maybe even a prince/princess). So Fairy Tail regroups and attacks the dark guild. There are many members of the guild, so there will be a lot of battling. I think it could be fun.