I have a few arc ideas:
1. Porygon & Cyberspace Arc - An arc, revolving around all three Porygon and cyberspace
2. The Grand Pokemon Tournament Arc - A grand tournament arc featuring Ash, Dawn, Brock, May, Gary, Paul, Drew, Harley, Solidad, Richie, Morrisson, Tyson, Harrison, Duplica, Conway, Damien(Ash's Charizard's former owner), Jun, Kenny, Nando, Zoey, Magikarp Salesman, Angie, Tracy, Forrest(Brock's younger brother), Anabel, etc. All in one huge Pokemon tournament.
3. Paul's Guild Arc - Paul forms an alliance with Conway, Harley, Damian, Dario(from the Ponyta episode), the Pokemon Bros. (Kim, Kai, & Kam), and the upper classmen students from the Pokemon Technical Institute(Season 1), where they steal the strongest pokemon of each person's party. This continues from the previous arc.
4. Global Team Rocket Arc - All of the Team Rocket organizations(from Giovanni, to Magma/Aqua, to PMC, to Team Galactic, to J) start an all out war against all pokemon trainers, particularly Ash; Paul defects to Team Rocket(Giovanni).
5. Satan Darkrai Arc - Darkrai regains his memories of his evil origin self. He begins his reign to bring the Pokemon World to darkness; he manages to manipulate both Mewtwo(Pokemon Movie) and Deoxys; After the failed Global Team Rocket invasion attempt, Meowth grows morose and joins up with Darkrai. This arc consists of the pokemon sages, almost every legendary(especially Arceus), all gym leaders as well as the elite four members, a plethora of Spiritomb, and Ash's Pikachu: The destined one.
EDIT: I'm sorry, I thought this was an idea thread for everyone.