Banner would probably be good if the CD was reduced

making one minion immune to magic damage doesn't strike me as needing the same cooldown as the aoe heal nuke . Would it really be too powerful if the CD was reduced to 60, or 75? In this case, you could just give it very high magic damage reduction instead of complete immunity, if it becomes too spammable, so that an AP champion isn't completely nullified by it, but instead has their push slowed to a crawl.

or we could just make it a general item that makes one minion or multiple minions in a wave extra durable, seeing as most AD champions now have potent waveclear, and if they don't they can buy it (and the fact that Banner has more armor than MR but the active is anti-AP)