My understanding at the time (I realize I could be mistaken about any of these):
* Users have no idea what code they're installing. Extensions aren't required to be open source, where the community can audit them for malicious behaviour. Even if an extension claimed to be open source, there's no verification system to ensure the code actually being executed is the code displayed on their github.
* Automatic updates. Maybe this isn't the case now, but at one point I remember extensions were updating themselves automatically. Users of popular extensions are frequently contacted to add an "analytics" dependency from some shady company, in return for a nice payout. Users don't have the ability to opt out of these kinds of updates when some of their extension devs inevitably cave into the pressure. In my mind, these kinds of updates should only be shipped with user's full consent and understanding that they are being asked to install updates which have no practical utility to them
* Code obfuscation. I don't see why every extension shouldn't be shipped fully unobfuscated, at least as an option. Perhaps minified bundles could also be shipped as a way of supporting users on low-spec devices which really need those extensions, but then again, if disk space is so short for those users maybe they shouldn't be installing extensions in the first place
* Better observability of interactions between user-requested websites and extension background pages. If I look in my network panel I want to see communication with installed extensions happening