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This is what I'd go with. Three pokemon from each gen, one from each type. Also those who don't have megas - give them mega and a fighting type. That would make...
Ursaring - normal/fighting
Marowak - ground/fighting
Barbaracle - rock/fighting
Golurk - fighting/ghost
Electivire - electric/fighting
Beartic - ice/fighting
Druddigon - dragon/fighting
Granbull - fairy/fighting
It would also allow a lot of fun gameplay styles thanks to pokes like Hawlucha, Barbaracle, Marowak, Breloom, Toxicroak, Druddigon