Hi Kai, thanks for this. I just wanted to reply to a few of your points. In your list here you say "option to [do something]" but none of the changes are actually options, so they can't be "implemented fast". I'll reply to them in order:
1. This has been done on the dev branch.
2. Your change always allows answers on closed questions so this is insecure.
3. I moved the HTML for that into the theme, so it can be overridden. The default is a standard list.
4. Your change seems to remove nofollow entirely which is bad for spam. You can remove nofollow in the theme, and do it conditionally, too.
5. This would require an admin option to use HTML email, and also the ability to put in custom code for each type of email.
6. I'll check into this but it seems you've restricted the elements too much.
7. That appears to be already done in 1.6.
8. Again, a better solution would be to have admin options for what types of changes are shown.
9. Not sure that's necessary, and can be done with themes anyway.
10. Why? That's what the All Activity page is for.
11. IMO this may be more confusing. Better to show them a notice when they click answer.
12. Most of this is doable via filter plugins. When a post is edited, check the details according to your own rules.
13. The button order is consistent already as far as I can tell.
14. Unless I am not understanding your point, this is already the case. The answer form is not on the page if the visitor is not logged in.
15. That's quite a useful idea.
16. Good idea, but your change hides it entirely. I may set it to hide for logged out users.
17. Will probably implement the idea mentioned by others, to hide users from the users page if they have no points.
18. Don't think this matters.
19. Nice idea, again it would need an actual admin option. (It should be possible to do it in your theme as well, I did at one point.)
20. Yes this makes sense. I'm looking into options to fully disabled selected pages (e.g. the users page) so this would go along with that.
Hope that's useful! You have some good ideas here. By the way, a lot of your core hacks can be done in plugins, so you should look into that - it would make upgrading much easier :)