Here's my idea for an anime exclusive story arc. It takes place in between the Avatar arc and the Alvarez Empire arc.
As the reunited Fairy Tail rebuild their guildhall, they receive a visit from former Phantom Lord members Boze and Sue, asking for their help. According to them, Jose Porla, who recovered from his fight with Makarov 8 years ago, is reforming Phantom Lord and they're planing to overthrow the newly formed Magic Council, with a captured Makarov as their hostage. Phantom has new members including an Iron Dragon Slayer who calls himself "The Son of Metalicana". This aggravates Gajeel. Fairy Tail's members decide to go stop them and save their master. They enter the new Phantom Lord guildhall the same way they entered the last one. To Fairy Tail's shock, Phantom never had Makarov, nor do they know where he is. The whole thing was a ruse to lour them into a trap. A guilty Sue uses Mirror Wall to trap them in and Phantom Lord's members, including their new strong ones, take out all of Fairy Tail's members, with the exception of Natsu, who is unaffected by Aria's Airspace Magic and defeats him. Aria asks Natsu how he beat him and Natsu says that there's always someone stronger. Happy points out that Natsu said the same thing Aria said to him when he kicked him to the curve and the two start arguing whether it was karma or a cruel irony. Gajeel attacks Boze for stabbing him in the back. To Gajeel's surprise, Boze enters Dragon Force, revealing himself to be the mysterious Iron Dragon Slayer; a 2nd Generation like Laxus and Cobra who had a Dragon Lacrima implanted in him. Boze then knocks a stunned Gajeel cold with Iron Dragon's Hard Fist and everyone else with Sound Wall. When they come to, Gajeel is missing. Sue and Aria reveal Phantom Lord's real plan. Jose spent the last 8 years rebuilding the 'R' System and needs Gajeel to resurrect Metalicana. Their plan is to hack into the Magic Council headquarters and fire the Etherion on the "Phantom Tower" and then place Gajeel inside the lacrima and fuse it's magic power with Gajeel's to reconstruct Metalicana's body. Once he's resurrected, they'll use Dragon Supremacy Magic to control him.
- Focus mostly on Gajeel
- Gajeel's past with Metalicana; when he first joined Phantom Lord
- Gajeel entres Dragon Force
- A fight between Natsu and Jose
You guys wanna hear more? It's not complete, so forgive me if it doesn't make sense. Everything but the rebuilt R-system and Metalicana is what I had in mind.