Black Manta realises he needs help to take down Arthur and his friends. He hijacks a prison transport plane headed for Belle Reve in his flying Manta -craft and recruits them to be his LEGION OF DOOM. He kills some minor villains who won't join him and promises riches and freedom for those who do.
Dunno About Lex, Biz and Nygma, Sinestro but howabout :
Toyman : creepy genius inventor.
Captain cold
Grundy maybe (dunno about Giganta and Grodd)
Definitely Scarecrow - he could use fear gas to give Arthur hydrophobia.
Whoever joins, they beat Arthur badly. He loses a hand in the process...
YOUR thoughts and ideas on this? (Needs work I know!)