It would actually be really cool if users could opt out of being sent to Posts right after logging in. Its a small and simple change that by giving the option, users can avoid seeing new untagged posts that can contain really unpleasant stuff. They could be sent to the front page instead for example. I dont want to see stuff that I hate right after I log in into here, and neither do others.

Im thinking it would affect how the site treats you after logging in, whether being you get sent to Posts or another part of e6. And the option to change the starting page would be in settings.

It's somewhat possible already, though not very convenient.

You can define the url parameter on the login page and it'll take you to that page after a successful login. For example, this one will take you to the homepage after logging in: https://e621.net/session/new?url=%2F

It can be any page on the site, it just needs to be URL encoded - e.g. /users/1187446 becomes %2Fusers%2F1187446