Toad, Magneto, the twins, even Juggernaut have been on the X-men. 
 
So which Brotherhood of Evil Mutants characters were never on the X-men... and what would it take to get them there?

if by twins you mean quicksilver and wanda then you are mistaken. 
 
pyro was never a member although he DID reform before his death. 

i was never on the x-men...nvm brotherhood is horrible i chose x men anyday

Blob probably and he could maybe be on the X-men if he was ever defined as more than just a one dimensional character.

@Avenging-X-Bolt said:

if by twins you mean quicksilver and wanda then you are mistaken. pyro was never a member although he DID reform before his death.

Quicksilver was a member of X-Factor for a while.

Yup. I know. But that's an entirely different team.