A better email client.
Why are there no more improvements? For example: I have many email accounts but tend to write to each person from a specific email account, so it bugs me every time I have to choose the account to send the email from, if I already send an email to this person, why can't the email client use the same account as last time? It's a simple lookup, but would make a great product!
Where else can email clients or other daily used software improve?