This should be a completely differen't suggestion, and I have a suggestion for your suggestion.
The banners suck other than hanging up your wall. It's super true, we doubt that the enemy resistance thing will work (man I wish it was the same with boss Trophies).
But instead, we could have a rewarding system, The first achievement should give you the banner, after that the banners will become completely RNG based. Next achievement should be something cool, like 'zombie arm' or 'black lens' or just coins.
The rewards could be either a rare drop of the mob, or just coins. The coins are more likely in lesser situations, while the higher prizes will be a rare drop.
Actually this reply could become a thread by itself.
Anyways, the idea is good for people who don't like a clogged inventory while doing events. Like me.