The main difference would be that Koikatsu at the end of the day is just a sex scene watcher with some glorified "gameplay" and visual novel elements, while AA2 had some surprisingly deep mechanics, up to a point where people didn't care about the sex at all.
In Koikatsu the only thing you can do is raise girls love and fuck them, but there's no jealousy, there's no consequences for actions, there is nothing beyond just talking with them and giving them the dick, which would change some of their lines and reactions during sex but nothing else.
Meanwhile for AA2 each character would have their own AI based in traits and the virtue and sexual orientation sliders; then it would consider things like friendship, love, the vibe during conversations or the popularity of characters to decide when to do interactions. This would lead to situations where some characters would try to sabotage others, some would try to go around humilliating or "forcing" people, some others would be depressed and just go away, some would try to stop anything H for happening and telling the teacher, some others would stalk their waifu/husbando, or the most famous one would be the yandere trying to kill their love competition.
AA2 doesn't look like much at first glance, but the gameplay was very fun once you understood what was going on and you could create your own scenarios. For now AA3 looks like it has all the same options as vanilla AA2, except for the traits and sliders that we still don't know if they exist and I personally think they contributed a lot to the personality of characters.
