Wanting to get back into the game, I played back in 8.3, got to 9/12 mythic Nyalotha, but i'm looking to reroll to Mage and was wondering if anyone had a guild i could join/potential raid team to join when i get geared enough (usually takes me about 1.5-2 weeks to get geared in mythic 15 stuff), alliance or horde doesnt matter to me neither does realm.

You're better off just to start gearing up and find a guild right now that's progressed in mythic and does ALT runs in heroic so you can get a feel for the current raid and your class.
Tell them you've raided mythic in past xpacs and you'll be good to go on their alt runs and get carried for gear as well.
Right now finding a mythic group that's pretty progressed is tough since most guilds are already set rosters.