You can't. People want to be at the highest level. Even if you replace it with a gear score (which is basically what level is above 20) people will want to max it. There is also the tendency to only raid with those of equal or higher level. Again, this can't be avoided. It's not that people *need* to be 30, it's that (1) it gives them a sense of completion and (2) others tend to take 30s over 29s when presented the choice.
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You shouldn't have to do the Raid in order to get to Lv.30. You want to make the Raid special, put uber weapons like the Mythocast or Atheon's Epilogue in there but for Christs sake you can't put the only armour in the game that can get you to the max level in the Raid.