I don't know if this is possible but why not let players keep the items they get and reset the levels. I find it to be a good solution. ( I mainly would like to keep my legendary gear from the iron banner that would effect the gameplay because it is cosmetic)
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I'm pretty sure that's the whole point of getting the iron banner stuff. Plus they're giving out exclusive emblems for people that play on the 26th, so it wouldn't make sense for them to wipe the characters after doing that.