Fandom.com has a nice article about X-Men which briefly talks about the villians in X-Men 2.
[X-Men Producer] DeSanto"s personal darling is Arcade, although he says the foe is more suitable for a comic one-shot. The Hellfire Club"s agenda and multiple members are too reminiscent of the Brotherhood. And at the mention of the Brood, DeSanto jokes, "I think Fox would sue us for ripping off Aliens." Singer does love the deadly Proteus but claims Juggernaut would just be too clunky. Which leaves...
"The Sentinels!" exclaims Singer. "The villains would be mankind."