Exactly the type of apps people would sideload. Little things that somehow violate App Store rules abusing APIs, lying about app capability, just being ethically dubious.

Use flash to create seizures, nudity people realtime, hack your ex, damage the device, cheat at games, spam your enemies, etc.

There is an infinite use case for tiny malicious apps finding malicious or gullible users and with side loading there are going to be stores created to appear very legitimate when their intentions are actually illegal.

I think the EU has very noble intentions while completely failing to understand that society is a wreck and a lot of money is made through extortion and fraud. Their apparent fix is to make the OS developers still responsible for what is installed while taking away funding for it. I am guessing the end game is more taxes and government intrusion on private devices to fix the problems they are purposely creating.