A lot of people seem very concern about being "forced" to do both LFR and Flex in addition to normal/heroic.
I propose "raid tokens". Enter and save to normal? One token gone. Enter LFR or Flex? Another token gone.
You get 2 tokens a week. This means you can do normal/heroic + LFR or Flex (but not both as you would need 3 tokens for that) or LFR + Flex (can't do normal/heroic for the week).
Problem solved.