Be able to create another account under the same email.

I agree. My brother accidentally made his account a girl so now he has to delete that account to make a new one under that email.

but how u think this should work? like - even on social media sites like instagram or twitter, idk..you need different emails, how do u want to decide in which profile u wanna log in? plus, most people with more profiles want to be online with 2 or 3 of them the same time

Yeah, I see the same issues. No sites really do this other than Google, but the way they do it is also being only logged in one at a time.

Yeah I had been thinking about this. The only way they would differentiate, is by passcodes. Maybe once you enter your email, it asks "which account would you like to log in to" idk. You could select it I guess. 

Or maybe you can use disposable mails? That's what I do, but you should remember the mail so that you can log in.

If you've used a Gmail to sign up, you can sign up again using the same mail. Instead of finishing with Gmail, do Googlemail instead. Goes to the same inbox :)

Little insider tip: 
If your current email is example@gmail.com and you want to create another account (let's call her Lisa), you can use example+lisa@gmail.com and all mails from this account will still  go to example@gmail.com (;