It would be nice to keep the EXP bar after level 20, perhaps having it track the next upgrade in your subclass or if you're maxed in that the nearest upgrade for your gun. At least showing how much EXP you're earning would be nice.
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I agree I've been thinking about this too. Not sure the best way it could be done. Even if it just had the xp show with no bar it would be nice. In halo campaigns I would turn on the score keeper thing just to know when I did get kills because on harder difficulties it's nice to know for a fact if you killed that vandal that just went around a corner