I'm not sure if this is already an option, but I think that having an option to save mail would be a good idea in case there is anything that you need to remember from the inbox and you run out of space for it. Since it is also difficult to prove if you have been scammed, harassed or bullied without image proof of the mail, this would also help protect users from those that wish to be toxic towards them!

About the space, however, I also believe that having a slightly larger inbox would be beneficial for those that often make there deals or have conversations with friends through inbox to have a bit more space within it. From personal experience, I have been having discussions with friends only to realize that either I have no more space in the box and have to decide which ones to get rid of or am unable to send anything to a friend due to them not having space......

I apologize if I come off as being either hot-headed or too venty with this idea