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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Add a rank system similar to halo 2s, where in the crucible, winning and losing means advancing, and declining in rank, halo 2s went up to 50, if you won you gained "exp", lost, you lost said "exp" you could be a rank 25 one game, then lose 5 and end up at 24. Boom, problem solved.